This family includes cytochrome b561 and related proteins, in addition to the nickel-dependent hydrogenases b-type cytochrome subunit. 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 prokaryotic family. Members of the 'eukaryotic cytochrome b561' family can be found in pfam03188.