RPE (R-Phycoerythrin), a fluorescent protein, belongs to the phycobiliproteins family, a family of highly soluble and fluorescent proteins derived from cyanobacteria and eukaryotic algae. RPE consists of a, b and g subunits and is present as (ab)6g. It has broad absorption bands with peaks at 565 nm (eM 1.96 X 106 M-1cm-1), 498 (eM 1.53 X 106 M-1cm-1), and 539 nm (eM 1.62 X 106 M-1cm-1), therefore can be excited with versatile excitation sources. RPE and the closely related BPE are the most intensely fluorescent phycobiliproteins and 10-20 times brighter than conventional organic fluorophores. Phycobiliproteins are also more photostable than their organic counterparts. RPE labeled streptavidin is widely used in flow cytometry, live cell staining, and multi-color immunofluorescent staining.