NADH-cytochrome b5 reductase involved in endoplasmic reticulum stress response pathway. Plays a critical role in protecting pancreatic beta-cells against oxidant stress, possibly by protecting the cell from excess buildup of reactive oxygen species (ROS). 2 Fe(III)-[cytochrome b5] + NADH = 2 Fe(II)-[cytochrome b5] + H(+) + NAD(+) Soluble protein. Ubiquitously expressed. Isoform 2 is expressed in testis, brain, skeletal muscle and in the male germline. Mice display insulin-deficient diabetes. Embryos and fetus develop normally. At 4 weeks of age, mice show have normal blood glucose levels but impaired glucose tolerance. Isolated islets have markedly impaired glucose- or arginine-stimulated insulin secretion. By 7 weeks of age, they develop severe hyperglycemia with markedly decreased serum insulin levels and nearly normal insulin tolerance. As the animals age, there is a progressive loss of beta cells in pancreatic islets, but there is no loss of alpha, delta, or PP cells. 4 week-old mice have enhanced sensitivity to the diabetogenic agent streptozotocin. Belongs to the flavoprotein pyridine nucleotide cytochrome reductase family. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus.