Allophycocyanin (APC) is a phycobiliprotein isolated from Spirulina sp., a blue-green alga. Like other phycobiliproteins, APC is fluorescent, with an extremely high absorptivity and high quantum efficiency. It is a protein which can be easily linked to antibodies and other proteins by conventional protein cross-linking techniques without altering its spectral characteristics.

Allophycocyanin is the least stable among the major phycobiliproteins, susceptible to dissociation at low concentrations including concentrations at which some assays are performed. To address this issue, AAT Bioquest offers CL-APC. This variant is chemically cross-linked between α and β subunits; it is much more stable than APC and has improved stability in aqueous solution.

For an example antibody conjugation protocol, please see here.