Haemoglobin Constant Spring—A Chain Termination Mutant ?

Haemoglobin Constant Spring is an unusual variant which comprises only 1–2% of the total haemoglobin in heterozygotes. It has α-chains which are 172 residues long instead of the normal 141, and when it is inherited together with an α-thalassaemia gene it gives rise to haemoglobin H disease.