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