Cytochrome b561 is a secretory vesicle-specific electron transport protein. It is an integral membrane protein, that binds two heme groups non-covalently. This is a eukaryotic family. Members of the 'prokaryotic cytochrome b561' family can be found in Pfam: PF01292.

1: Structural basis for the electron transfer across the chromaffin vesicle membranes catalyzed by cytochrome b561: analyses of cDNA nucleotide sequences and visible absorption spectra. Okuyama E, Yamamoto R, Ichikawa Y, Tsubaki M; Biochim Biophys Acta. 1998;1383:269-278. PMID:9602148