Phycochromes are a subclass of phycobiliprotein that was initially recognised only as light sensory pigments in cyanobacteria.  They are now deemed to constitute of all possible photoreversibly photochromic pigments, regardless of function. They are also found in red algae. In a series of journal articles written by G.S. and L.O. Björn, it was reported that phycochromes a, b, c and d were discovered by scientists who fractionated samples of blue-green algae using electrofocusing. The fractions with isoelectric points at or around 4.6 seemed analogous to phytochromes in that they possessed photochromic properties, yet were sensitive to light of shorter wavelengths.  All four phycochromes except phycochrome c were extracted from the blue-green algae Tolypothrix distorta; whereas phycochrome a was also found in Phormidium luridum, Nostoc muscorum 1453/12 and Anacystis nidulans; and phycochrome c was extracted from Nostoc muscorum A and Tolypothrix tenuis.