Abnormal haemoglobin formation occurs in these conditions, which are hereditary haemolytic anaemias. The haemoglobin may be abnormal because: (1) there is a defect in the synthesis of normal adult haemoglobin as in thalassaemia, when there may be an absence of one or both of the polypeptide chains that characterise normal adult haemoglobin; or (2) there is an abnormal form of haemoglobin such as haemoglobin S, which results in sickle-cell disease. This abnormality may involve as little as one amino acid of the 300 in the haemoglobin molecule. In sickle-cell haemoglobin, one single amino-acid molecule that of glutamic acid is replaced by another that of valine. This results in such a deficient end product that the ensuing disease is frequently severe.