Mouse mammary tumour virus (MMTV)/neu transgenic mice develop clonal or oligoclonal mammary tumours stochastically. The pathology of these tumours is very similar to that of human breast tumours. Moreover, these mouse tumours metastasize in the lungs. We present evidence that this mouse model of human breast tumours can be instrumental in identifying novel genes of two distinct classes (activated oncogenes or tumour suppressor genes) which may collaborate with the c-erbB-2/neu transgenic oncogene.