APC (Allophycocyanin), a near-infrared fluorescent protein, belongs to the phycobiliproteins family of highly soluble and fluorescent proteins derived from cyanobacteria and eukaryotic algae. APC is greater than 100 times more sensitive than conventional organic fluorophores. APC is made up of alpha and beta subunits and is present as a trimer (??3). The trimer is unstable and susceptible to dissociation at low concentrations. A chemically cross-linked APC trimer (APC-XL) is much more stable than the native APC trimer, but still retains comparable spectroscopic properties as the native APC trimer. APC labeled streptavidin is widely used in flow cytometry, live cell staining, and multi-color immunofluorescent staining.