Hence, the primary cause of inherited methemoglobinemia is a congenital deficiency in cytochrome b5 reductase and is inherited in an autosomal recessive pattern.2 Hemoglobin M disease is the other form of congenital methemoglobin and is inherited as an autosomal dominant defect.5,6 In Hemoglobin M disease, a mutation in the gene coding for one of the globin chains results in a substitution of a tyrosine amino acid for either the proximal (F8) or the distal (E7) histidine amino acid in the α, β, or γ chains. This evidence concerns the gene HBG2 and methemoglobinemia.