Aplasia or haemolysis may be precipitated by another illness/infection in patients with sickle cell disease. Acutely severe anaemia is defined as Hb < 5 g/dl or a recent acute drop in HB > 2 g/dl below the individual’s steady state value. If no steady state value is available, it can be detected by the presence of acutely symptomatic anaemia (i.e. tachycardia, cardiac failure, shock). The gene discussed is GSTM1; the disease is sickle cell disease.