Electron donor reductase for cytochrome b5. The cytochrome b5/NADH cytochrome b5 reductase electron transfer system supports the catalytic activity of several sterol biosynthetic enzymes (By similarity). 2 Fe(III)-[cytochrome b5] + NADH = 2 Fe(II)-[cytochrome b5] + H(+) + NAD(+) Monomer. Belongs to the flavoprotein pyridine nucleotide cytochrome reductase family.