Quantitative Mcm Perturbation in Mice Figure 4. Premature morbidity and cancer susceptibility in Mcm4Chaos3/Chaos3 Mcm2Gt/+ mice. (A) Kaplan-Meier survival plot of the indicated genotypes. Animals of both sexes are combined. ‘‘C3’’ = Chaos3. (B) Spleen and liver histopathology of a Mcm4C3/C3 Mcm2Gt/+ male diagnosed with T cell leukemic lymphoma. i. H&E stained spleen. Neoplastic cells have abundant cytoplasm, 1–2 nucleoli and a high mitotic rate, consistent with lymphoblastic lymphoma. Bar = 20 mm. ii. Neoplastic cells in spleen demonstrate immunoreactivity with anti-CD3 (brown; immunoperoxidase staining with DAB chromogen & hematoxalin counterstain), indicating T lymphocytes. Bar = 200mm. iii. In spleen, immunoreactivity (brown) with anti- PAX-5 (a B cell marker) is limited to follicular remnants and scattered individual cells. Bar = 200 mm. iv. In liver, neoplastic cells surround central veins and expand sinusoids (see also Figure S4) and demonstrate immunoreactivity (brown) with the anti-CD3 T lymphocyte marker. Bar = 50 mm. doi:10.1371/journal.pgen.1001110.g004 Mcm4Chaos3/Chaos3 Mcm2Gt/+ Mcm3Gt/+ survivors compared to Mcm4Chaos3/Chaos3 Mcm2Gt/+ animals produced from the same matings (Figure 5B). Mcm3 hemizygosity also significantly rescued the near 100% lethality of Mcm4Chaos3/Gt animals (nearly 6 fold increased viability), and doubled the viability of Mcm4Chaos3/Chaos3 Mcm6Gt/+ mice (Figure 5A; Figure S3). Rescue of Mcm4Chaos3/Chaos3 Mcm7Gt/+ was not observed (not shown). The rescue of the reduced growth phenotype by Mcm3 hemizygosity led us to evaluate the proliferation of compound mutant cells. Whereas Mcm4Chaos3/Chaos3 and Mcm4Chaos3/Chaos3 Mcm3Gt/+ primary MEFs proliferated at identical rates, Mcm4Chaos3/Chaos3 Mcm2Gt/+ MEFs showed a severe growth defect beginning ,5 days in culture (Figure 5C). As with whole animals, MEF growth was partially but significantly rescued by Mcm3 hemizygosity. Since the Mcm4Chaos3 and Mcm2Gt alleles causes elevated GIN (micronuclei in RBCs), we considered the possibility that the Mcm3 rescue effect might be related to an attentuation of GIN. Accordingly, PLoS Genetics | www.plosgenetics.org 7 September 2010 | Volume 6 | Issue 9 | e1001110