High frequencies of host erythrocyte polymorphisms such as α+-thalassaemia, haemoglobin (Hb) S, Hb C, Hb E, complement receptor-1 (CR1) deficiency, glucose-6-phosphate dehydrogenase (G6PD) deficiency and south-east Asian ovalocytosis (SAO) are found in malaria endemic areas. The gene discussed is CR1; the disease is malaria.