Mouse Mammary Tumor Virus (MMTV) is primarily associated with mammary carcinoma and lymphomas in mice. The signal peptide of MMTV-Env precursor (MMTV-p14) translocates to nucleoli of infected cells and co-localizes with nucleophosmin. Mutations along the sequence of MMTV-p14 ectopically expressed in MCF-7 breast carcinoma cells affect cellular localization of the protein in vitro (mutations within the nuclear localization signal – NLS) and tumorigenicity in vivo (mutations in putative phosphorylation sites). Immunofluorescence of MMTV-p14 with half of the NLS deleted (green) demonstrates partial localization in the nucleus and the cytoplasm. Nucleophosmin (red) remains in the nucleoli. For details, see article by Feldman and colleagues on page 1077.