In a 2.5-month-old infant with beta-thalassaemia major, DNA analysis of the gamma-beta region revealed homozygosity for two large deletions removing the entire psi beta and beta 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 delta-globin gene to about 1.8 kb on the 3' side of the beta-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 psi beta-globin gene to about 4.5 kb on the 3' of the psi beta gene. The delta-globin gene was intact. From the phenotypic expression of the disease it is concluded that removal of the psi beta gene probably prevents derepression of the gamma gene that has previously been observed in the absence of the promoter region of the beta gene and the switch mechanism from gamma to beta gene expression may take place earlier than expected.