DNA replication is the most essential event in the propagation of a species. Defects in this process can cause diseases including birth (developmental) defects and cancer. When DNA replication is compromised genetically or environmentally, a state known as “replication stress” (RESS) occurs that can lead to the aforementioned deleterious outcomes. Extensive basic research on the biochemistry of DNA replication has been conducted in single celled organisms and cultured cells, without regard for potential gender differences that may exist in higher organisms such as mammals. The goal of this project is to understand how the universal process of DNA replication is subject to dramatic sexual dimorphism in mammalian embryogenesis. Preliminary studies found that female mouse embryos were dramatically more prone to lethality when levels of the MCM2-7 (Minichromosome maintenance 2,3,4,5,6,7) DNA replication licensing and helicase proteins were genetically reduced and the helicase destabilized. Subsequent studies revealed that the female-biased lethality began occurring immediately after sex determination, and was not related to defects in X-inactivation. Transgene-mediated conversion of female embryos to males or testosterone administration reversed the gender-biased lethality, indicating that the phenomenon is related to secondary sexual characteristics. Further experiments suggested that testosterone enabled female embryo rescue by virtue of its anti-inflammatory activity, a possibility supported by the observation that ibuprofen, a non-steroidal anti-inflammatory drug (NSAID), also rescued MCM-deficient female embryos. This project seeks to elucidate the cellular and mechanistic basis of this novel example of mammalian sexual dimorphism. Aim 1 will test whether female- biased embryo lethality in MCM-depleted animals is due to gender differences in DNA replication per se, or differential sensitivity to intrinsic RESS. Aim 2 will test the hypothesis that female embryo hypersensitivity is related to RESS-induced inflammation, while males embryos are protected by the anti-inflammatory activity of testosterone they produce in high levels following sex determination. Aim 3 will explore the basis of preliminary data showing that a key contributing factor in the sex bias phenomenon is the maternal environment; only dams with intrinsic RESS preferentially lost female embryos. This will be accomplished transferring at-risk genotypes of zygotes into foster mothers that are genotypically wild-type, MCM-deficient, or deficient for the anti-inflammatory cytokine IL-10. Overall, these studies will can impact our understanding of the relationships between RESS, inflammation, and adverse pregnancy outcomes related to maternal and/or fetal inflammatory responses that are genetically or environmentally induced.