Phycobiliproteins are a family of highly soluble and reasonably stable fluorescent proteins derived from cyanobacteria and eukaryotic algae. These proteins contain covalently linked tetrapyrrole groups that play a biological role in collecting light and, through fluorescence resonance energy transfer, conveying it to a special pair of chlorophyll molecules located in the photosynthetic reaction center.ref Because of their role in light collection, phycobiliproteins have evolved to maximize both absorption and fluorescence and to minimize the quenching caused either by internal energy transfer or by external factors such as changes in pH or ionic composition.ref Phycobiliproteins have several advantages when used as fluorescent probes, including: