The human fragile X mental retardation 1 (FMR1) gene contains a polymorphic (CGG) trinucleotide repeat element in its 5' untranslated region. Expansion of the (CGG)n element beyond 200 repeats (full mutation range) generally leads to transcriptional silencing; consequent loss of the FMR1 protein (FMRP) results in fragile X syndrome, the most frequent form of inherited mental impairment. For carriers of smaller expansions (55< or =n< or =200; premutation range), FMRP levels are gradually reduced with increasing repeat number, despite elevated FMR1 mRNA levels, suggesting that translation is impeded within the premutation range. To examine in more detail the influence of the CGG repeat on translation, CMV immediate-early promoter constructs, containing the FMR1 5'-UTR with various (CGG)n repeat lengths (0< or =n< or =99) and a downstream (luciferase) reporter, were transfected into two human cell lines, a neural cell-derived line (SK) and a fetal kidney cell-derived line (293). For both cell types, the CGG element exerts distinct effects on reporter expression, depending on the length of the repeat. For n> or =30, luciferase expression decreases with increasing repeat length, consistent with earlier observations of decreased FMRP expression in peripheral blood leucocytes over the same repeat range, despite a slight increase in mRNA level for the larger repeats. Surprisingly, for smaller alleles (0< or =n< or =30), reporter expression actually increases by nearly two-fold with increasing repeat length in the absence of any change in mRNA level. These results suggest that the CGG repeat element can exert both positive (n<30) and negative (n>30) effects on translation. Interestingly, optimal translation appears to occur near the modal repeat number within the general human population.