Hemoglobin consists of one heme unit (a complex of one iron atom within four protoporphyrin structures) plus one globin unit (consisting of two pairs of polypeptide chains, one pair known as alpha and the other pair called beta). The heme units are identical in each hemoglobin molecule; changes in amino acid sequence in the globin chains result in different hemoglobins.

The major hemoglobin structural abnormalities can be divided into two groups: the hemoglobinopathies, consisting of amino acid substitutions in the globin beta chain; and the unstable hemoglobins, which have focal structural abnormality in the globin chains of a different type than the hemoglobinopathies, predisposing the hemoglobin molecule to denaturation.