Cytochrome b558 is comprised of two subunits, gp91phox and p22phox. The large subunit actually encodes a protein of about 65 kDa, but glycosylation causes it to run as a diffuse band around 91 kDa on SDS gels. gp91phox contains a region of homology with the NADPH-binding region of several flavoenzymes including cytochrome P450 reductase and ferredoxin-NADP+ reductase, and also show weaker homology with the FAD binding site of several flavoprotein dehydrogenases. The figure also shows point mutations in variants of Chronic Granulomatous Disease which inhibit NADPH oxidase activity as well as proposed mechanisms in some cases.