A common mutation causing thalassemia in Mediterranean populations is an amber (UAG) nonsense mutation at the 39th codon of the human beta-globin gene, the beta-39 mutation. Studies of mRNA metabolism in erythroblasts from patients with beta-39 thalassemia and studies using heterologous transfection systems have suggested the possibility that this mutation not only affects protein synthesis but also alters mRNA metabolism. The effects of this mutation on several steps in the metabolism of mRNA have been investigated by transfection of the gene into permanent cell lines bearing a temperature-sensitive RNA polymerase II. Several RNA expression studies were performed, including analysis of transcription, mRNA stability, mRNA splicing accuracy, and mRNA polyadenylation. The results suggest that the defect in expression of the beta-39 mRNA occurs at a step prior to the accumulation of mRNA in the cytoplasm.