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Haemoglobin S is a haemoglobin variant (valine replaces glutamic acid in the sixth position of the [3-chain) which forms a gel-like structure when deoxygenated. This causes reversible and eventually irreversible changes in red cell morphology.Patients who are homozygous for HbS (sickle cell disease -SCD) have significant illnesses and shortened life expectar~cy, but those who are heterozygous (sickle cell trait -SCT) have lower amounts of haemoglobin S (25-45 per cent of total haemoglobin) and are usually thought to be free of in viva sickling and to be basically

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