Frequency of very severe anaemia (Hb < 5 g/dL) among G6PD-deficient participants receiving 0.25 mg/kg of primaquine was very rare (≤ 1/10,000) if baseline haemoglobin was ≥ 10 g/dL, rare (between 1/1000 and 1/10,000) if baseline haemoglobin was 9 g/dL, and uncommon (between 1/100 and 1/1000) if baseline haemoglobin was 8 g/dL. The gene discussed is G6PD; the disease is anemia (phenotype).