From these studies and from a screen performed in mice transgenic for activated rat ErbB2 (Supplementary Data 1; GJI, MB, ERMB, JH, unpublished data), we obtained MMTV proviral insertion data from a total of 1,132 tumours, of which 35 (3.1%) had insertions that map in the Irs4-locus on the X-chromosome. This evidence concerns the gene ERBB2 and neoplasm.