To determine whether differential expression of the mammalian Notch receptors NOTCH1 and NOTCH2, which are particularly associated with different breast cancer subtypes with different prognosis [11,12,13,14], is found in mouse mammary tumours of different histotypes and may, therefore, contribute to the generation of tumour heterogeneity, we analysed receptor expression using qrtPCR in tumours from two different genetic backgrounds (previously reported elsewhere) [1]. The gene discussed is NOTCH1; the disease is breast cancer.