NOTCH1 and NOTCH4 pro-viral integration sites mediate mammary tumour formation (Dievart et al, 1999; Gallahan & Callahan, 1987), Notch4 is upregulated in T-ISC in primary non-invasive (Farnie et al, 2007) and invasive ER positive breast cancers (Harrison et al, 2010) and Notch1 overexpression in breast cancer correlates with worse prognosis (Reedijk et al, 2005; Stylianou et al, 2006). Here, NOTCH1 is linked to breast cancer.