Summary. Determinants of mRNA stability are frequently positioned in the 3′UTR where they are not subject to disruption by actively translating ribosomes. Two related individuals with β thalassaemia who carry a β-globin gene containing a 13 nt deletion in its 3′UTR have recently been described. Its position within the 3′UTR, as well as its relative distance from other known functionally important elements, suggested that the deletion might overlay previously unrecognized determinants of β-globin mRNA stability. We studied the impact of the Δ13 mutation on β-globin gene expression in vitro and in vivo. The adverse effect of the Δ13 mutation on β-globin expression was confirmed in studies utilizing reticulocytes from a βΔ13 heterozygote, which indicated a sixfold reduction in the relative level of the mutant mRNA. Additional in vitro analysis indicated that the deletion did not affect the capacity of the βΔ13 mRNA to assemble an mRNA-stabilizing mRNP ‘β-complex’. Unexpectedly, functional tests in both primary erythroid cells and in a transgenic mouse model demonstrated that the βΔ13 mRNA was fully stable, suggesting that the Δ13 mutation affects accumulation of the fully processed mRNA at an earlier step. Consistent with this, there was a relative excess of unprocessed βΔ13 mRNA in erythroid progenitors from a βΔ13 heterozygote. Taken together, these results define a new thalassaemic determinant, which acts to decrease β-globin mRNA levels by inhibiting the efficiency of nuclear processing events, and suggest a previously unanticipated complexity to the role of the 3′UTR elements in the regulation of β-globin gene expression.