Human and rodent trans-species carcinogens often demonstrate similar organotropic patterns of neoplasia and loss of heterozygosity (LOH)[See Figure 1 and Table 1 below]. Recently, we have observed significant chromosome 11 LOH in B6.129-Trp53tm1Brd N5 mice heterozygous for a p53 null allele using simple sequence length polymorphic loci analysis (see Figure 2). Primers specific for known SSLP loci revealed amplicons consistent with the two strains, C57BL/6 and 129Sv, present in the mice. The degree of heterozygosity observed on chromosome 11was unexpected because the mice were reported to be on a C57BL/6-Trp53 (N5) background which would be approximately 3% 129Sv alleles primarily flanking the null allele (129 embryonic stem cell origin). By exploiting the observed heterozygosity on chromosome 11 in the 5th backcross generation, we learned that LOH was not restricted to theTrp53 locus as determined by Southern analysis of genomic DNA from tumors and control tissue. A complete copy of chromosome 11 of maternal origin carrying the p53 wildtype allele was lost during exposure to phenolphthalein resulting in thymic lymphomas in these mice. The investigation confirmed a chromosome 11 non-dysjunction mechanism of action for phenolphthalein as revealed by allelotype analysis based on comparison to the germline pattern of SSLP loci. Chromosome 11 loss also occurred in benzene and p-cresidine induced p53 (+/-) mouse sarcomas (oral, intubation) and thymic lymphomas (inhalation, whole animal) and bladder tumors (dietary), respectively. The results establish microsatellite (SSLP loci) mapping as a useful tool for determination of LOH in tumor studies using p53 haploinsufficient mice. We hypothesize that carcinogen induced DNA damage in the p53 haploinsufficient mice resulted in either chromosome 11 mis-segregation or recombination during induced repair processes that induced LOH resulting in genomic instability leading to neoplasia. In summary, we have shown that in independent studies that there is sufficient heterozygosity on chromosome 11 in the heterozygous p53 deficient (+/-) N5 generation mouse to use microsatellite markers at 5 cM intervals to demonstrate whole or partial chromosome loss through non-disjunction and homologous recombination.