GSTM1 and Cyanosis: Congenital cyanosis can be caused by hereditary methemoglobinemia due to either NADH-methemoglobin reductase (NADH-MR) deficiency (Percy and Aslan, 2008; Percy and Lappin, 2008), or the presence of an abnormal hemoglobin (Hb-M) (Da-Silva et al., 2003; Kedar et al., 2005).