The nucleotide sequence of three single alpha-globin genes resulting from a rightward 3.7-kb deletion is described. The alpha genes were isolated from the DNA of three subjects homozygous for this deletion, the first being in addition homozygous for the structural mutation alpha G Philadelphia (genotype -alpha G/-alpha G), the second, heterozygous for this structural mutation (genotype -alpha A/-alpha G) and the third homozygous for an alpha + -thalassaemic gene (genotype -alpha +thal/-alpha +thal). The latter subject produced HbH in contrast to the two others. Whereas the two alpha A and alpha G genes are identical to the normal alpha 1-globin gene (except for the alpha G point mutation), the alpha +thal gene has (i) a deletion of the two nucleotides at position -2 and -3 preceding the ATG codon, and (ii) a fusion between the 5' part of the normal alpha 2 gene and the 3' part of the normal alpha 1 gene. Using a dot-blot assay, we show that reticulocytes from the HbH subject contain at least as much alpha mRNA as reticulocytes from the two other subjects. In a transient expression system, the alpha +thal gene leads to normally spliced transcripts. We conclude from these data that the defective output of alpha chains by the alpha +thal gene, as evidenced by HbH production, results from a decreased efficiency of alpha-mRNA translation due to the two nucleotides deletion preceding the AUG codon.