The FreeStyle™ 293 Expression System

Introduction

The FreeStyle™ 293 Expression System is designed to allow large-scale transfection of suspension 293 human embryonic kidney cells in a defined, serum-free medium. The system includes FreeStyle™ 293-F cells that have been adapted to serum-free, suspension culture in FreeStyle™ 293 Expression Medium. Transfection and
expression experiments may be performed directly in FreeStyle™ 293 Expression Medium without the need to change media. The complete FreeStyle™ 293 Expression Kit provides enough reagents to perform 16 transfections in a 30 ml volume, but larger volume transfections may be performed using simple scale-up of reagents.

Components of the FreeStyle™ 293 Expression System

The FreeStyle™ 293 Expression System includes the following major components:

  • FreeStyle™ 293-F cells: This cell line is adapted to high density, serum-free suspension culture in FreeStyle™ 293 Expression Medium and is capable of producing high levels of recombinant protein (see more information below).
  • FreeStyle™ 293 Expression Medium: This is a defined, serum-free medium formulated specifically to allow growth and large-scale transfection of suspension FreeStyle™ 293-F cells.
  • 293fectin™: This transfection reagent provides high transfection efficiency in suspension FreeStyle™ 293-F cells.
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Ordering Information

Sku Name Size Price Qty
K900001 FreeStyle™ 293 Expression System 1 kit USD 927.50
R79007 FreeStyle™ 293-F Cells 1 mL USD 457.92
12338018 Gibco® FreeStyle™ 293 Expression Medium 1,000 mL USD 101.00
12338026 Gibco® FreeStyle™ 293 Expression Medium 6 x 1 L USD 542.00
12347019 293fectin™ Transfection Reagent 1 mL USD 417.00
31985062 Opti-MEM® I Reduced Serum Medium 100 mL USD 18.30
31985070 Opti-MEM® I Reduced Serum Medium 500 mL USD 41.60
K210004 PureLink® HiPure Plasmid Midiprep Kit 25 preps USD 240.72
K210006 PureLink® HiPure Plasmid Maxiprep Kit 10 preps USD 211.14
K210016 PureLink® HiPure Plasmid Filter Maxiprep Kit 10 preps USD 230.52
K210008 PureLink® HiPure Plasmid Megaprep Kit 4 preps USD 247.86
15250061 Trypan Blue Solution, 0.4% 100 mL USD 14.90
10586014 Plasmid pCMV•SPORT-βgal 25 µg USD 430.00
K145501 β-Gal Assay Kit 80 mL USD 253.00
K900010 FreeStyle™ MAX 293 Expression System 1 kit USD 1,038.00
K900020 FreeStyle™ MAX CHO Expression System 1 kit USD 1,104.00
R80007 FreeStyle™ CHO-S Cells 1 mL USD 495.00
12651022 Gibco® FreeStyle™ CHO Expression Medium 6 x 1,000 mL USD 481.00

FreeStyle™ 293-F Cells

Introduction

The FreeStyle™ 293-F cell line is supplied with the FreeStyle™ 293 Expression System and is derived from the 293 cell line (see below). FreeStyle™ 293-F cells are adapted to suspension culture in FreeStyle™ 293 Expression Medium. Frozen cells are supplied in and may be thawed directly into FreeStyle™ 293 Expression Medium (see Thawing and Establishing Cells).

Parental Cell Line


The 293 cell line is a permanent line established from primary embryonal human kidney transformed with sheared human adenovirus type 5 DNA (Graham et al., 1977; Harrison et al., 1977). The E1A adenovirus gene is expressed in these cells and participates in transactivation of some viral promoters, allowing these cells to produce very high levels of protein.

The FreeStyle™ 293-F cell line supplied with the FreeStyle™ 293 Expression System is a variant of the 293 cell line that has been adapted to suspension growth in FreeStyle™ 293 Expression Medium. The 293-F cell line was obtained from Robert Horlick at Pharmacopeia.

Characteristics of FreeStyle™ 293-F Cells

The FreeStyle™ 293-F cell line exhibits the following characteristics:

  • Prepared from low passage Master Cell Bank cultures derived from parental 293-F cells that were re-cloned by limiting dilution. The 293 clonal derived cultures are maintained in serum-free conditions for only 30 to 35 total passages.
  • Adapted to high density, serum-free, suspension growth and are maintained in FreeStyle™ 293 Expression Medium.
  • Demonstrates high transfection efficiencies with 293fectin™.
  • Suspension cultures may be transfected in FreeStyle™ 293 Expression Medium without the need to change media.
  • Permits transfection of cells at large volumes. 

Note:  Other 293 cell lines may be used with the FreeStyle™ 293 Expression System. Before these cell lines may be used for transfection studies, however, they must be adapted to serum-free, suspension culture in FreeStyle™ 293 Expression Medium and evaluated for transfection and expression.
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FreeStyle™ 293 Expression Medium

Introduction

FreeStyle™ 293 Expression Medium is a defined, serum-free medium specifically developed for the high-density, suspension culture and transfection of 293 cells. The medium contains NO human or animal origin components.

Features of the Medium

FreeStyle™ 293 Expression Medium exhibits the following features:

  • An optimized, serum-free, protein-free formulation designed to support the high-density culture and transfection of 293 cells (e.g. FreeStyle™ 293 cells) in suspension. The medium is not recommended for adherent 293 cell culture.
  • Prepared ready-to-use, with no supplementation required.
  • Contains no human or animal-origin products.
  • Formulated with Glutamax™-I (see below) to increase stability and maximize shelf life.
  • Supports the large-scale, high-density growth of FreeStyle™ 293-F cells in bioreactors.

Glutamax™-I

Glutamax™-I media contain the dipeptide, L-alanyl-L-glutamine, a stabilized form of L-glutamine. With Glutamax™-I media:

  • L-glutamine does not degrade in storage or during incubation
  • Ammonia build-up is minimized
  • Glutamine delivery is controlled
  • L-glutamine does not need to be added at the time of use.

Note: Glutamax™-I is only removed from the medium by cell metabolism. There is no accumulation of toxic metabolites due to spontaneous breakdown.

Growth Characteristics of FreeStyle™ 293-F Cells in the Medium

Typically, FreeStyle™ 293-F cells cultured in FreeStyle™ 293 Expression Medium demonstrate the following:

  • Doubling time in the range of 20-25 hours Note: The doubling time can exceed 25 hours during the first few passages after the cells have been thawed.
  • Cell densities of up to 3 x 106 cells/ml in shaker or spinner culture
  • Cell densities of up to 4 x 106 cells/ml in bioreactor culture

Note:
Individual culturing and passaging techniques coupled with cellular heterogeneity inherent within the FreeStyle™ 293-F cell population may result in experimental variability.
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Methods

General Information

General Cell Handling

Follow the general guidelines below to grow and maintain FreeStyle™ 293-F cells.

  • All solutions and equipment that come in contact with the cells must be sterile. Always use proper sterile technique and work in a laminar flow hood.
  • Before starting experiments, be sure to have cells established and also have some frozen stocks on hand. We recommend using early-passage cells for your experiments. Upon receipt of the cells from Invitrogen, grow and freeze multiple vials of the FreeStyle™ 293-F cell line to ensure that you have an adequate supply of early-passage cells.
  • For general maintenance of cells, pass FreeStyle™ 293-F cells when they reach a density of approximately 1-3 x 106 viable cells/ml (generally every 3-4 days).
  • Use trypan blue exclusion to determine cell viability (see below). Log phase cultures should be >90% viable.
  • When thawing or subculturing cells, transfer cells into pre-warmed medium.



Caution:   As with other human cell lines, when working with FreeStyle™ 293-F cells, handle as potentially biohazardous material under at least Biosafety Level 2 containment.

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Preparing Media

For suspension growth and transfection applications, use:

  • FreeStyle™ 293 Expression Medium as is. No supplementation is required.
  • Antibiotics are not recommended; however, 5 ml/L of Antibiotic-Antimycotic (Catalog no. 15240) containing penicillin, streptomycin, and amphotericin B may be used when required.


Important:  FreeStyle™ 293 Expression Medium is extremely sensitive to light. For optimal results, use and store media protected from light.

Determining Cell Density and Viability

Follow the procedure below to determine viable and total cell counts.

  1. Transfer a small aliquot of the cell suspension to a microcentrifuge tube.
  2. Determine viability and the amount of cell clumping using the trypan blue dye exclusion method.
  3. Vigorously vortex for 10-30 seconds to break up cell clumps.
  4. Determine cell density electronically using a Coulter Counter or manually using a hemacytometer.


Thawing and Establishing Cells

Introduction


Follow the protocol below to thaw FreeStyle™ 293-F cells to initiate cell culture. The FreeStyle™ 293-F cell line is supplied in a vial containing 1 ml of cells at 1 x 107 viable cells/ml in 90% FreeStyle™ 293 Expression Medium and 10% DMSO. Thaw FreeStyle™ 293-F cells directly into the FreeStyle™ 293 Expression Medium supplied with the kit.

Materials Needed


You will need to have the following reagents on hand before beginning:

  • FreeStyle™ 293-F cells (supplied with the kit; store frozen cells in liquid nitrogen until ready to use)
  • FreeStyle™ 293 Expression Medium (supplied with the kit; pre-warmed) Note: We do not recommend adding antibiotics to media as this may negatively impact cell growth.
  • 125 ml polycarbonate, disposable, sterile Erlenmeyer flask
  • Orbital shaker in 37°C incubator with a humidified atmosphere of 8% CO2
  • Room temperature table-top centrifuge and sterile centrifuge tubes
  • Reagents to determine viable and total cell counts
  • Sterile, 50 ml conical tubes


Thawing Procedure

Store frozen cells in liquid nitrogen until ready to use. To thaw and establish cells:

  1. Remove the cryovial of cells from the liquid nitrogen and thaw quickly in a 37°C water bath.
  2. Just before the cells are completely thawed, decontaminate the outside of the vial with 70% ethanol. Triturate and transfer the entire contents of the cryovial into a 125 ml polycarbonate, disposable, sterile Erlenmeyer shaker flask containing 17 ml of pre-warmed FreeStyle™ 293 Expression Medium.
  3. Incubate cells in a 37°C incubator containing a humidified atmosphere of 8% CO2 in air on an orbital shaker platform rotating at 125 rpm. Loosen the cap of the flask a quarter turn from snug to allow oxygenation/aeration.
  4. Once the culture has reached greater than 1 x 106 viable cells/ml (typically 3 to 5 days), transfer the cell suspension aseptically into a centrifuge tube and vortex for 10 seconds.
  5. Determine viable and total cell counts.
  6. Subculture the FreeStyle™ 293-F cells by seeding shaker flasks at 3 x 105 viable cells/ml in pre-warmed FreeStyle™ 293 Expression Medium. We generally use 125 or 250 ml polycarbonate, disposable, sterile, Erlenmeyer flasks containing 20 or 40 ml total working volume of cell suspension, respectively.


Important Note: Subculture cells a minimum of two additional times before use in transfection experiments to allow opportunity for recovery from thawing. To subculture cells, see the procedure below.

Subculturing Cells

Passaging Cells


Subculture cells when the density is approximately 2-3 x 106 viable cells/ml, typically every 3-4 days. When maintaining FreeStyle™ 293-F cells, we generally use a 125 or 250 ml polycarbonate, disposable, sterile Erlenmeyer flask containing 25 to 40 ml or 50 to 80 ml total working volume of cell suspension, respectively. Note: Glass flasks without baffles may be used, but thorough cleaning after each use is essential to avoid potential toxicity which is more problematic in serum-free cultures.

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  1. Determine viable and total cell counts.
  2. Using the cell density determined in Step 1, calculate the split ratio needed to seed the new shaker flask at 3 x 105
  3. Dilute the cells in fresh, pre-warmed FreeStyle™ 293 Expression Medium to give a final cell density of 0.1-0.2 x 106 viable cells/ml in the desired final volume.
  4. Incubate flasks in a 37°C incubator containing a humidified atmosphere of 8% CO2 in air on an orbital shaker platform rotating at 135 rpm.
  5. Repeat Steps 1-5 as necessary to maintain or expand cells. Monitor the degree of cell clumping (see below).


Recommendation:  FreeStyle™ 293-F suspension cultures may grow as 2 to 10 cell clusters. Vigorous vortexing for 10-30 seconds may be required at each subculture for a number of passages until the cultures grow predominantly as single cells.

Scaling Up Cell Culture

It is possible to scale up the FreeStyle™ 293-F cultures in spinner flasks or bioreactors. The appropriate spinner or impeller speed and seeding density should be determined and optimized for each system. At Invitrogen, the optimum spinner speed was 100-130 rpm and 70-100 rpm impeller speed in Celligen™  stirred tank bioreactors. We recommend seeding cells at 3-5 x 105 viable cells/ml.

Note: If the split ratio of cells to fresh media is less than  1:2, you may want to spin down the cell suspension and resuspend the cell pellet in fresh, pre-warmed FreeStyle™ 293 Expression Medium prior to inoculating the spinner or bioreactor culture. Monitor cell viability and the degree of cell clumping. Note that extensive cell clumping may reduce transfection efficiency.

Recommendation: 
  At high stirring speeds (i.e. greater than 130 rpm) and/or depending on the impeller design, you may want to supplement the FreeStyle™ 293 Expression Medium with additional Pluronic® F-68 (2.5-5 ml/L of 10% Pluronic® F-68, Catalog no. 24040) to avoid sheer stress in the culture.

Pluronic® is a registered trademark of BASF Corporation. Celligen™ is a registered trademark of New Brunswick Corp.

Freezing Cells

Introduction

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You may freeze FreeStyle™ 293-F cells directly in FreeStyle™ 293 Expression Medium. When freezing the FreeStyle™ 293-F cell line, we recommend the following:

  • Freeze cells at a density of 5-8 x 106 viable cells/ml.
  • Use a freezing medium composed of 90% fresh FreeStyle™ 293 Expression Medium and 10% DMSO. Guidelines to prepare freezing medium and to freeze cells are provided in this section.


Preparing Freezing Medium

Prepare freezing medium immediately before use.

  1. In a sterile, conical centrifuge tube, mix together the following reagents for every 1 ml of freezing medium needed:  FreeStyle™ 293 Expression Medium 0.9 ml DMSO 0.1 ml
  2. Filter-sterilize the freezing medium and place the tube on ice until use. Discard any remaining freezing medium after use.


Freezing Cells

Before starting, label cryovials and prepare freezing medium. Keep the freezing medium on ice.

  1. Grow the desired quantity of FreeStyle™ 293-F cells in shaker flasks, harvesting when the cell density reaches 0.5 to 1 x 106 viable cells/ml. Transfer cells to a sterile, conical centrifuge tube.
  2. Determine the viable and total cell counts and calculate the volume of freezing medium required to yield a final cell density of 5-8 x 106 viable cells/ml.
  3. Centrifuge cells at 100 x g for 5 minutes at room temperature and carefully aspirate the medium.
  4. Resuspend the cells in the pre-determined volume of chilled freezing medium.
  5. Place cryovials in a microcentrifuge rack and aliquot 1 ml of the cell suspension into each cryovial.
  6. Freeze cells in an automated or manual, controlled-rate freezing apparatus following standard procedures. For ideal cryopreservation, the freezing rate should be a decrease of 1°C per minute.
  7. Transfer frozen vials to liquid nitrogen for long-term storage.


Note: You may check the viability and recovery of frozen cells 24 hours after storing cryovials in liquid nitrogen by following the procedure outlined in Thawing and Establishing Cells

Transfecting Cells

Introduction


To transfect suspension FreeStyle™ 293-F cells, you will use the cationic lipid-based transfection reagent, 293fectin™ included with the kit. Unlike some other serumfree media formulations, FreeStyle™ 293 Expression Medium does not inhibit cationic lipid-mediated transfection. FreeStyle™ 293 Expression Medium is specifically formulated to allow high efficiency transfection of suspension FreeStyle™ 293-F cells without the need to change or add media. Transient transfection experiments may be performed in a large volume, allowing larger-scale protein production.

293fectin™

293fectin™ is a proprietary formulation suitable for transfection of nucleic acids into eukaryotic cells. In the FreeStyle™ 293 Expression System, use of 293fectin™ to transfect FreeStyle™ 293-F cells provides the following advantages:

  • 293fectin™ demonstrates high transfection efficiency in suspension FreeStyle™ 293-F cells (cultured in FreeStyle™ 293 Expression Medium)
  • DNA-293fectin™ complexes can be added directly to cells in culture medium
  • It is not necessary to remove complexes or change or add medium following transfection 293fectin™ is available  separately from Invitrogen . For more information, see our Web site (www.invitrogen.com) or call Technical Service.



Opti-MEM® I

Opti-MEM® I Reduced Serum Medium is included with the FreeStyle™ 293 Expression System to facilitate optima  formation of DNA-293fectin™ complexes. Opti-MEM® I is a modification of Eagle’s Minimal Essential Medium, buffered with HEPES and sodium bicarbonate, and supplemented with hypoxanthine, thymidine, sodium pyruvate  L-glutamine, trace elements, and growth factors. The protein level is minimal (15 μg/ml) with insulin and transferrin being the only protein supplements. Phenol red is included at a reduced concentration as a pH indicator. Opti-MEM® I Reduced Serum Medium is available separately from Invitrogen.


Positive Control

pCMV SPORT-βgal is provided as a positive control vector for transfection and expression in FreeStyle™ 293-F cells. The gene encoding β-galactosidase is expressed in FreeStyle™ 293-F cells under the control of the human cytomegalovirus (CMV) promoter. Successful transfection will result in β-galactosidase expression that is easily assayed. For a map of pCMV SPORT-βgal.

Assay for β-galactosidase Activity

You may evaluate β-galactosidase expression by activity assay using cell-free lysates (Miller, 1972). Invitrogen offers the β-Gal Assay Kit (Catalog no. K1455-01) for fast and easy detection of β-galactosidase expression.

Plasmid Preparation

Plasmid DNA for transfection into eukaryotic cells must be clean, sterile and free from phenol and sodium chloride. Contaminants may kill the cells, and salt will interfere with complexing, decreasing transfection efficiency. We recommend isolating plasmid DNA using one of the Purelink™ HiPure Plasmid Kits (Catalog no. K2100-14 or K2100-16).

Note:  Make sure your DNA preparation is sterile, for instance by performing filtration through a 0.22 μm filter before use.

Materials to Have on Hand

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You will need to have the following reagents on hand before beginning:

 

  • Suspension FreeStyle™ 293-F cells cultured in FreeStyle™ 293 Expression Medium Recommendation: Calculate  the number of cells that you will need for your transfection experiment and expand cells accordingly. Make sure that the cells are healthy and greater than 90% viable before proceeding to transfection.
  • Purified plasmid DNA of interest
  • 293fectin™ (supplied with the kit; store at +4°C until use)
  • Opti-MEM® I Reduced Serum Medium (supplied with the kit; pre-warmed)
  • FreeStyle™ 293 Expression Medium (supplied with the kit; pre-warmed) Note: Do not add antibiotics to media during transfection as this may decrease transfection activity.
  • 125 ml polycarbonate, disposable, sterile Erlenmeyer flasks
  • Orbital shaker in 37°C incubator with a humidified atmosphere of 8% CO2
  • Room temperature table-top centrifuge and sterile, conical centrifuge tubes
  • Reagents to determine viable and total cell counts
  • Sterile, disposable, polycarbonate snap-cap tubes
  • Vortex mixer


Optimal Conditions for 30 ml Transfection

We generally perform transfection experiments in a 30 ml volume. To transfect suspension FreeStyle™ 293-F cells, we recommend using the following optimized conditions:

  • Final transfection volume: 30 ml
  • Number of cells to transfect: 3 x 107 cells (final cell density of 1 x 106 cells/ml)
  • Amount of plasmid DNA: 20-40 μg (we typically use 30 μg)
  • Amount of 293fectin™: 40-80 μl (we typically use 60 μl). Use 2 μl 293fectin™ per 1 μg of plasmid DNA transfected


Note:   If you are using other 293 cells, you may want to test varying amounts of 293fectin™ (e.g. 30, 40, 50, 60, 80 μl) with 30 μg plasmid DNA to determine the optimal conditions for transfection.

Transfection Procedure

Follow the procedure below to transfect suspension FreeStyle™ 293-F cells in a 30 ml volume. Remember that you may keep the cells in FreeStyle™ 293 Expression Medium during transfection. We recommend including a positive control (pCMV SPORT-βgal) and a negative control (no DNA, no 293fectin™) in your experiment.

  1. The day before transfection (day 1), determine the number of cells that you will need for your experiment. Remember that for each 30 ml transfection, you will need 3 x 107 cells in 28 ml of FreeStyle™ 293 Expression Medium. Tip: To transfect cells on day 2, seed cells at a density of 6~7 x 105 viable cells/ml. To transfect on day 3, seed cells at a density of 3~4 x 105 cells/ml.
  2. On the day of transfection, transfer a small aliquot of the cell suspension to a microcentrifuge tube and determine viability and the amount of cell clumping using the trypan blue dye exclusion method. Vigorously vortex for 45 seconds to break up cell clumps and determine total cell counts using a Coulter Counter or a hemacytometer. Viability of cells must be over 90%. Important: For optimal transfection results, make sure that you have a single cell suspension. It may be necessary to vortex the cells for 10 to 30 seconds.
  3. Calculate the volume of cell suspension containing the number of cells needed for one transfection (for each 3  ml transfection, you will need 3 x 107 cells). Place the shaker flask containing cells in a 37°C incubator on an orbital
  4. For each transfection sample, prepare lipid-DNA complexes by performing the following:
  5. Dilute 30 μg of plasmid DNA in Opti-MEM® I to a total volume of 1 ml. Mix gently.
  6. Dilute 60 μl of 293fectin™ in Opti-MEM® I to a total volume of 1 ml. Mix gently and incubate for 5 minutes at room temperature. Note: Longer incubation times may result in decreased activity.
  7. After the 5 minute incubation, add the diluted DNA to the diluted 293fectin™ to obtain a total volume of 2 ml. Mix gently.
  8. Incubate for 20-30 minutes at room temperature to allow the DNA- 293fectin™ complexes to form.
  9. While the DNA-293fectin™ complexes are incubating, remove the cell suspension from the incubator and add the appropriate volume of cell suspension (see step 3) into each sterile, disposable 125 ml Erlenmeyer shaker flask. Add fresh, pre-warmed FreeStyle™ 293 Expression Medium up to a total volume of 28 ml for a 30 ml transfection.
  10. After the DNA-293fectin™ complex incubation is complete, add the 2 ml of DNA-293fectin™ complex to each shaker flask from Step 4. To the negative control flask, add 2 ml of Opti-MEM® I instead of DNA-293fectin™ complex. Each flask should contain a total volume of 30 ml, with a final cell density of approximately 1 x 106 viable cells/ml
  11. Incubate the cells in a 37°C incubator with a humidified atmosphere of 8% CO2 in air on an orbital shaker rotating at 125 rpm.
  12. Harvest cells or media (if recombinant protein is secreted) at approximately 48 hours post-transfection and assay for recombinant protein expression.


Optimizing Protein Expression

Expression levels may vary depending on the nature of your recombinant protein; therefore, you may want to perform a time course (i.e. harvest cells or media at 24, 48, 72, 96 hours post-transfection) to optimize expression of your recombinant protein.

Scaling Up Transfections

It is possible to perform transfection experiments in a larger (e.g. 1 liter) volume. If you wish to transfect suspension FreeStyle™ 293-F cells in a larger volume, scale up the volume of each reagent accordingly. The table below lists suggested conditions to use when transfecting FreeStyle™ 293-F cells in a 1 liter or 3.8 liter volume. The optimized conditions to use when transfecting FreeStyle™ 293-F cells in a 30 ml volume are listed as a reference. Note that transfection conditions may vary depending on the type of culture vessel used and the growth conditions of your cells; therefore, you may want to perform pilot studies to optimize your transfection conditions.

Transfection Volume Total Number of Cells* Amount of DNA DNA Dilution Volume (in Opti-MEM® I) Amount of 293fectin™ 293fectin™ Dilution Volume (in Opti-MEM® I) Lipid/DNA Complex Volume
30 ml3 x 10730 μgto 1 ml60 μlto 1 ml2 ml
1 liter1 x 1091 mgto 35 ml2 mlto 35 ml70 ml
3.8 liter3.8 x 1093.8 mgto 125 ml7.6 mlto 125 ml250 ml


Note: The transfection efficiency may decrease as the volume increases if the FreeStyle™ 293-F cells are not growing as a single-cell suspension (i.e. if significant cell clumping is observed).

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References

  1. Graham, F. L., Smiley, J., Russell, W. C., and Nairn, R. (1977). Characteristics of a Human Cell Line Transformed by DNA from Human Adenovirus Type 5. J. Gen. Virol. 36, 59-74.

  2. Harrison, T., Graham, F., and Williams, J. (1977). Host-range Mutants of Adenovirus Type 5 Defective for Growth in HeLa Cells. Virology 77, 319-329.

  3. Miller, J. H. (1972). Experiments in Molecular Genetics (Cold Spring Harbor, New York: Cold Spring Harbor Laboratory).
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25-0439    Version C   15-Mar-2007