CYB5R3 and cardiomyopathy, familial restrictive, 1: Recessive congenital methemoglobinemia (RCM) (OMIM #250800) is an autosomal disease resulting from mutations in the CYB5R3 gene, which encodes NADH-cytochrome b5 reductase 3 (NADH-CYB5R3).