Crossing the Muc1 knockout mouse (Muc1−/−) with strains expressing mouse mammary tumor virus (MMTV)-driven oncogenes suggests that Muc1 cooperates with other tumorigenic factors in cancer: MMTV-Wnt-1 and MMTV-TGFα mice show significantly delayed onset of tumors when on the Muc1−/− background (Schroeder et al. 2003; Pochampalli et al. 2007), while MMTV-PyV MT (polyomavirus middle T antigen) mice lacking Muc1 have significantly reduced tumor progression with a trend towards decreased metastasis (Spicer et al. 1995). The gene discussed is MUC1; the disease is neoplasm.