A 22-year-old African woman first presented to our medical outpatients department in 1993 with anaemia, with test results as follows: haemoglobin (Hb) 96 g/litre (normal range 120 to 160 g/litre), mean corpuscular volume (MCV) 74.7 fl (normal range 79 to 95 fl), mean corpuscular haemoglobin concentration (MCHC) 313 g/litre (normal range 320 to 360 g/litre), reticulocytes 13‰ (normal range 5 to15‰) and ferritin of 9 ng/ml (normal range 10 to 200 ng/ml). The gene discussed is GSTM1; the disease is anemia (phenotype).