The light-harvesting antenna system in red algae and cyanobacteria is formed of phycobilisomes. These are composed of the phycobiliproteins phycoerythrin (CPE), phycocyanin (CPC) and allophycocyanin (APC). Cyanobacteria also contain phycoerythrocyanin (PCC). The phycobiliproteins all share the same subunit composition and organization with variations in the covalently bound open-chain tetrapyrrole chromophores. The phycobiliprotein complexes are arranged sequentially in antenna complexes linked by linker proteins with CPE at the periphery, CPC in the middle and APC at the core feeding to the photosynthetic reaction center.