PTPN1 and breast neoplasm: When PTP1B knockout mice were crossed with mouse mammary tumor virus (MMTV)-NeuNT mice, less than 40% of MMTV-NeuNT/PTP1−/− mice developed breast tumors over a 3 year period, whereas MMTV-NeuNT/PTP1B+/+ mice had tumors with an average latency of 13 months [5].