A rare red cell disorder classified principally into two clinical phenotypes: autosomal recessive congenital (or hereditary) types I and II (RCM/RHM type 1; RCM/RHM type 2). In RCM type 1, cyanosis from birth is the only symptom. RCM type 2 is much more severe; the cyanosis is accompanied by neurological dysfunction (with intellectual deficit, microcephaly, growth retardation, opisthotonus, strabismus and hypertonia), which usually becomes evident during the first four months of life. RCM type 1 is caused by mutations of the CYB5R3 gene (22q13.31-qter) encoding the NADH-cytochrome b5 reductase (Cb5R) and Cb5R deficiency is limited to the erythrocytes. RCM type 2 is caused by global loss of Cb5R function. RCM type 1 is generally associated with missense mutations, whereas RCM type 2 is more commonly associated with truncating mutations, splicing errors or mutations that lead to disruption of the active site.