NADH-cytochrome b5 reductases are involved in desaturation and elongation of fatty acids, cholesterol biosynthesis, drug metabolism, and, in erythrocyte, methemoglobin reduction. 2 Fe(III)-[cytochrome b5] + NADH = 2 Fe(II)-[cytochrome b5] + H(+) + NAD(+) Belongs to the flavoprotein pyridine nucleotide cytochrome reductase family. It is uncertain whether Met-1 or Met-2 is the initiator. Extended N-terminus.