The major function of cytochrome b561 is now clear. This transmembrane protein transports electrons across a secretory vesicle bilayer to supply electrons to monooxygenases inside the secretory vesicle. Cytochrome b561 has been localized not only to adrenergic secretory vesicles, where it supplies electrons to dopamine beta-hydroxylase, but also to peptidergic secretory vesicles that contain peptidyl alpha-amidating monooxygenase. Thus, one would expect to find cytochrome b561 in the membranes of all neuroendocrine cells that contain amidated peptide secretory products. In addition, its wide occurrence as an integral membrane protein of secretory vesicles may make it useful for investigation of vesicle biogenesis and turnover. One of the most important potential roles of cytochrome b561 is that it can be used as a model protein to investigate long-range biological electron transport. This cytochrome is a single polypeptide, which can be purified easily and reconstituted into a functional assembly. It also catalyzes an experimentally unambiguous transmembrane transport of electrons. A full molecular characterization of the structure and function of this cytochrome may provide insights into biological electron transfer which would otherwise be difficult or impossible to obtain.