An altered level of normal haemoglobin (Haemoglobin A (Hb A), Haemoglobin A2 (Hb A2) or Haemoglobin F (Hb F, fatal haemoglobin) in the blood, which may be seen in those suffering with sickle cell disease (SCD). This evidence concerns the gene GSTM1 and Schnyder corneal dystrophy.