Summary. In a 2.5-month-old infant with β-thalassaemia major, DNA analysis of the gamma-beta region revealed homozygosity for two large deletions removing the entire β and β regions including their 5’ promoter regions but leaving the delta gene intact. The downstream deletion was predicted to be 7.6 kb in length extending from a point 1.5 kb on the 3’ side of the δ-globin gene to about 1.8 kb on the 3’ side of the β-globin gene. The upstream deletion, which was also about 7.6 kb, extended from a point 1.5 kb on the 5’ side of the β-globin gene to about 4.5 kb on the 3’ of the β gene. The δ-giobin gene was intact. From the phenotypic expression of the disease it is concluded that removal of the β gene probably prevents derepression of the γ gene that has previously been observed in the absence of the promoter region of the β gene and the switch mechanism from gamma to beta gene expression may take place earlier than expected.