Increasing data strongly suggest that, in adults, the G gamma/A gamma globin chains ratio is genetically controlled by determinants linked in cis to the beta-globin gene cluster. The presence of the C----T substitution at position -158 5' to the G gamma globin gene in subjects displaying high G gamma/A gamma globin chains ratio led to the hypothesis that this substitution is involved in the determination of high G gamma globin gene expression. In order to test this hypothesis, we have compared the expression of two G gamma globin genes carrying a C or a T at position -158, upon transfection into a COS cell line or stable integration into a MEL cell line. Our results showed no significant effect of the nucleotide at position -158 on the level of human G gamma mRNA produced in these cells.