Cytochromes b₅ are ubiquitous electron transport hemoproteins found in animals, plants, fungi and purple phototrophic bacteria. The microsomal and mitochondrial variants are membrane-bound, while bacterial and those from erythrocytes and other animal tissues are water-soluble. The family of cytochrome b₅-like proteins includes (besides cytochrome b₅ itself) hemoprotein domains covalently associated with other redox domains in flavocytochrome cytochrome b₂ (L-lactate dehydrogenase; EC1.1.2.3), sulfite oxidase (EC1.8.3.1), plant and fungal nitrate reductases (EC1.7.1.1, EC1.7.1.2, EC1.7.1.3), and plant and fungal cytochrome b₅/acyl lipid desaturase fusion proteins.