Most eukaryotic messenger RNAs have the sequence AAUAAA 11-30 nucleotides from the 3'-terminal poly(A) tract. Since this is the only significant sequence homology in the 3' non-coding region it has been suggested that it may be a recognition site for enzymes involved in polyadenylation and/or termination of polymerase II transcription. This idea is strengthened by observations on the effect of deletion mutations in or around the AATAAA sequence on polyadenylation of late simian virus 40 (SV40) mRNA; removal of this sequence prevents poly(A) addition. Naturally occurring variants of this hexanucleotide are rare and hitherto their functional significance has not been assessed. We have now identified a human alpha 2-globin gene which contains a single point mutation in this hexanucleotide (AATAAA leads to AATAAG). The paired alpha 1 gene on the same chromosome is completely inactivated by a frame-shift mutation. This unique combination has enabled the expression of the mutant alpha 2 gene to be studied in vivo where it has been found that the accumulated level of alpha 2-specific mRNA in erythroid cells is reduced. Furthermore, readthrough transcripts extending beyond the normal poly(A) addition site are detected in mRNA obtained from HeLa cells transfected with cloned DNA from the mutant alpha 2 gene, suggesting that the single nucleotide change in the AATAAA sequence is the cause of its abnormal expression.

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The molecular basis of alpha‐thalassaemia has been established in 48 Thai subjects with Hb H disease and 15 with the Hb Bart's hydrops fetalis syndrome. This study has shown that in this population there are at least 18 different types of chromosome carrying seven independent alpha‐thalassaemia mutations one of which is a novel deletion removing the entire alpha‐globin gene complex. Although there are a limited number of alpha‐thalassaemia determinants in the Thai population, there is a remarkable degree of variation in the genetic markers which flank them. These markers may be of value in establishing the evolutionary history of the alpha‐thalassaemias.