By isolating genomic DNA clones that encompass the mouse Cyl-1 (cyclin D1) locus, we have identified a putative CpG island close to the 5'end of the gene. Pulsed-field gel electrophoresis with probes derived from either the 5'or 3'side of the CpG island established physical linkage to two independent markers on mouse chromosome 7, in a region that is syntenic with human chromosome 11q13. On the 3'side, Cyl-1 is approximately 75 kb from Hst-1 and Int-2, although there is an additional CpG island in the intervening DNA, while on the 5'side, Cyl-1 is less than 300 kb from Fis-1, an integration site for Friend murine leukaemia virus. As there is no intervening CpG island, proviral insertions at Fis-1 could influence the expression of Cyl-1 and we describe two virally induced tumours in which this appears to be the case. The data suggest that proviral insertions near Cyl-1 in mouse lymphomas are functionally equivalent to the BCL1 translocations that activate cyclin D1 expression in human B-cell malignancies.