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 (see ANAEMIA). 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.