Enables cytochrome-b5 reductase activity, acting on NAD(P)H. Acts upstream of or within several processes, including glucose homeostasis; insulin secretion; and response to antibiotic. Located in endoplasmic reticulum. Is expressed in metanephros; nervous system; sensory organ; submandibular gland primordium; and thymus primordium. Used to study type 2 diabetes mellitus. Human ortholog(s) of this gene implicated in type 2 diabetes mellitus. Orthologous to human CYB5R4 (cytochrome b5 reductase 4). [provided by Alliance of Genome Resources, Apr 2022]