Upon the B cell antigen receptor (BCR) ligation Ca2+ mobilization is induced, which is essential for activation of downstream signaling molecules such as MAP kinase. Although synthetic fluorescent chelators such as Fluo-4 and Indo-1 are widely used for Ca2+ measurement upon BCR ligation, they are leaked or unfavorably localized into some organelles with time post loading. To solve these problems, we introduce a genetically encoded fluorescent indicator cameleon which is a fluorescence resonance energy transfer (FRET)-based indicator comprising two fluorescent proteins (CFP and YFP) and two Ca2+-responsive elements (a variant of calmodulin (CaM) and a CaM-binding peptide). Here, we demonstrate that cameleon as well as a conventional synthetic Ca 21 indicator enables Ca2+ measurement by flow cytometry clearly upon BCR ligation. In addition, confocal microscopy analysis allows us to detect cameleon-based Ca2+ mobilization in a single cell upon BCR ligation. (c) 2007 Elsevier Inc. All rights reserved.