Notch genes (Notch1 and Notch4), when expressed under the control of whey acidic protein (WAP) or mouse mammary tumor virus (MMTV) promoters in the mouse, result in the formation of mammary carcinoma (Jhappan et al., 1992; Gallahan et al., 1996; Dievart et al., 1999). The gene discussed is NOTCH1; the disease is breast carcinoma.