Summary In this study we investigated the mechanism of stabilisation of IL-2 receptor α subunit mRNA by the HTLV-I protein p27rex. We tested the role of the nucleolar targetting signal in rex by introducing mutations. Three deletion mutants could not express rex protein in the nucleolus and although protein was still expressed in the nucleoplasm none of the mutants could stabilise IL-2Rα mRNA. A substitution mutant could be expressed in the nucleolus and could also stabilise IL2Rα mRNA. The data show that the nucleolar targetting signal is crucial for stabilisation of IL-2Rα mRNA by rex and raise the possibility that transport of mRNA from nucleus to cytoplasm can involve the nucleolus.