Transferrin is a monomeric serum glycoprotein (~80,000 daltons) that binds up to two Fe3+ atoms for delivery to vertebrate cells through receptor-mediated endocytosis.  1-3 Once iron-carrying transferrin proteins are inside endosomes, the acidic environment favors dissociation of iron from the transferrin–receptor complex.  Following the release of iron, the apotransferrin is recycled to the plasma membrane, where it is released from its receptor to scavenge more iron.  4,5 Fluorescent transferrin conjugates can therefore be used with fluorescent LDL to distinguish the lysosomally directed and recycling endosomal pathways.