This family contains the S-type phycobiliprotein (PBP) lyase (denoted CpcS/CpcU or CpeS/CpeU). PBP lyases are employed by cyanobacteria, red algae, cryptophytes and glaucophytes for light-harvesting. Pigmentation of light-harvesting phycobiliproteins of cyanobacteria and cryptophytes requires covalent attachment of open-chain tetrapyrrole chromophores, the phycobilins, to the apoproteins. PBP lyases mediate this covalent attachment of phycobilin chromophores to apo-PBPs and also ensure the correct binding of the chromophore with regard to the specific attachment site and stereospecificity. The S-type lyase is distantly related to CpcT and similarly adopts a beta-barrel structure with a modified lipocalin fold. Many members of the CpcS/CpcU family ligate phycocyanobilin (PCB) to a specific cysteine residue in the beta-subunits of phycocyanin (CpcB) or phycoerythrocyanin (PecB) and to a related cysteine residue in the alpha and beta subunits of allophycocyanin (AP); they are typically given the designation of "CpcS" or "CpcU". Other members which attach phycoerythrobilin (PEB) to the beta-subunit of phycoerythrin (PE) are given the designation "CpeS" or "CpeU". In Guillardia theta, a Cryptophyte, which has adopted phycoerythrobilin (PEB) biosynthesis from cyanobacteria, phycobiliprotein lyase has been shown to provide structural requirements for the transfer of this chromophore to the specific cysteine residue of the apophycobiliprotein.