Allophycocyanin (APC) is a 105 kDa phycobiliprotein found in cyanobacteria and red algae. It exhibits a bright far-red fluorescence with a molar extinction coefficient of 700,000 cm-1M-1 and a fluorescence quantum yield of 0.68. The fluorescence spectral of APC in its native state is characterized by two absorbance peaks, a primary absorbance peak at 651 nm and a secondary absorbance peak at 625 nm, and a single emission maximum at 660 nm. This dual absorbance maxima improves compatibility with imaging instruments equipped with red lasers (633-647 nm). Just like PE,APC must first be conjugated to a molecule having biological specificity, such as an antibody, streptavidin or annexin V prior to using. It offers several advantages that make it a popular choice for multicolor flow cytometry experiments:

• Long-wavelength emission profiles minimizes autofluorescence from biological materials
• Minimal fluorescence quenching contributed by the covalent binding of phycobilins to the protein backbone
• Very high water-solubility to facilitate chemical manipulation for conjugation reactions
• Significant Stokes shifts with resolvable emission spectra for multicolor analysis
• Multiple sites for stable conjugation with organic and synthetic compounds such as antibodies, cyanine dyes or iFluor® dyes
• Tandem conjugates for multicolor flow cytometry - generate a range of emission signals with a single excitation laser