A point mutation in the simian virus 40 large-T gene, which was generated by mixed oligonucleotide mutagenesis and resulted in the conversion of Lys 128 to Thr, produced a large-T antigen that was detected in the cytoplasm but not the nucleus of cells. Deletions within the surrounding sequence Lys- 128Lys-Lys-Arg-Lys-Val-Glu also produce cytoplasmic large-T and define a region of the protein involved in nuclear location. © 1984 Nature Publishing Group.