This cell-permeant indo-1, AM is a UV light—excitable, ratiometric Ca2+ indicator. The emission maximum of indo-1 shifts from ∼475 nm in Ca2+-free medium to ∼400 nm when the dye is saturated with Ca2+. This AM ester form of the dye is available in 1mg amounts (I-1203) and in 1 mM solution (I-1226). A cell-impermeant pentapotassium (F-1202) salt form of the dye is also available.