We are attempting to identify cellular oncogenes activated in mammary tumours by using the mouse mammary tumour virus (MMTV) as an insertional mutagen. MMTV, a retrovirus lacking a host cell-derived viral oncogene, induces adenocarcinomas of the mammary gland after a long latency period. The tumours are clonal outgrowths of cells carrying one or more integrated MMTV proviral copies. We have cloned an integrated MMTV provirus with its adjacent chromosomal DNA and we have established that the insertion site was part of a domain of the mouse genome in which MMTV proviruses are inserted in many different tumours. A gene within this domain, called int-1 is transcriptionally activated as a consequence of proviral integration. We have proposed that int-1 is a cellular oncogene for mammary tumours. Proviral activation of int-1 occurs in cis, over distances of up to 10 kilobases and is presumably caused by the transcriptional enhancer present on the MMTV long terminal repeat. The putative int-1 mammary oncogene has been subjected to a detailed structural analysis by S1 mapping and DNA sequencing. It encodes a protein that is highly conserved between mouse and man. The protein encoding domain of the gene is distributed over four exons which are demarcated by the insertion sites of MMTV proviruses found in mammary tumours. Some insertions, however, are found in the transcriptional unit of int-1, but these insertions do not disrupt the protein encoding domain of the gene.