Fluo-3FF is a fluorescent calcium indicator. It is a di-fluorinated analog of Fluo-3 that displays about a 100-fold lower affinity than Fluo-3 for calcium (Kds = 42 and 0.4 μM, respectively). Low affinity calcium indicators are particularly useful for studying compartments with high concentrations of calcium, such as endoplasmic reticulum, where high affinity dyes will be insensitive to luminal fluctuations. Fluo-3FF has excitation/emission maxima of approximately 506 and 526 nm, respectively.