The 5' untranslated region of an RNA molecule is thought to play an important role in the regulation of translation, Following a recent report that a single nucleotide is sufficient to act in this role in the unicellular organism Giardia, we show that this is also the case for a mammalian in vitro system. These results also demonstrate that an RNA can initiate translation from a start codon where an ideal translational consensus sequence is impossible. (C) 1997 Federation of European Biochemical Societies.