p53 is one of the most frequently mutated genes in mammary carcinomas (MCs). To detect tumor suppressor genes cooperating with a hetero-deficient p53 gene in mammary carcinogenesis, we first examined allelotypes in MCs from (BALB/cHeA x MSM/Ms) F(1)- p53(+/-) and (BALB/cHeA x 129/SvEv) F(1)- p53(+/-) female mice, and then surveyed down-regulated genes in the allelic loss regions. Genome-wide screening at 42 loci identified frequent (more than 30%) loss of heterozygosity (LOH) on chromosomes 5, 8, 11, 12, 14 and 18 in the MCs from either of the F(1) mice. The MCs in the p53(+/- )mice indicated highly frequent LOH, especially on chromosomes 8, 11 and 12, distinct from other mouse tumors. More than 60% of the 38 MCs from (BALB/cHeA x MSM/Ms) F(1)- p53 (+/-) mice showed LOH in a region ranging from D8Mit85 (105.0 Mb from centromere) to D8Mit113 (111.8 Mb) on chromosome 8, a region syntenic to human chromosome 16q22.1, on which LOH has been found in breast cancers. RT-PCR analyses revealed that the LOH of chromosome 8 was associated with the reduced and/or complete loss of expression of Cdh1 and Cdh5 genes in 15 (58%) and 8 (31%) of 26 MCs derived from the F(1) mice, respectively. Thus, inactivation of Cdh1 and Cdh5 is likely to cooperate with the loss of p53, suggesting a possible tumor suppressive function of these genes in mammary carcinogenesis.