HBM and hereditary methemoglobinemia: Hereditary methemoglobinemia linked to CYB5R3 mutations causes nicotinamide adeninedinucleotide (NADH)-cytochrome-reductase deficiency, while autosomal dominant disorders involving various globin-encoding genes lead to HbM formation, characterized by structural abnormalities, causing iron auto-oxidation and methemoglobinemia.