NOTCH1 and breast cancer: For example, the constitutively activated form of Notch acts as a bona fide oncogene in leukaemia and in murine breast cancer, while recent data show that Notch-1 can function as a tumour suppressor gene in skin cancer (Ellisen et al, 1991; Uyttendaele et al, 1996; Nicolas et al, 2003).