Thalassaemia intermedia: a new molecular basis.

A 5-year-old child heterozygous for beta thalassaemia has the clinical picture of thalassaemia intermedia. Restriction endonuclease mapping shows that the child is homozygous for a triplicated alpha gene complex. The greater degree of globin chain imbalance resulting from two additional alpha chain genes is the likely mechanism for this unusually severe clinical phenotype.