Most mammary carcinomas induced in C3H mice by the mouse mammary tumour virus (MMTV) bear a new proviral insertion within a highly conserved locus on chromosome 15 called int-1. A transcriptional unit within this locus is inactive in all tested normal tissues but expressed at low levels in mammary tumours with proviral insertions positioned on either the 5' and 3' sides of the gene. Transcription of the proviruses proceeds away from int-1; thus an indirect mechanism appears to activate expression of this putative oncogene.