The long-term goal of this proposal is to determine whether aging is associated with perturbations in the balance of double strand break (DSB) repair by homologous recombination (HR). DSBs accumulate in aging and senescent cells, and homozygous mutations in HR genes such as those causing Bloom's syndrome, Werner syndrome or Fanconi anemia are associated with premature aging, age-related disease and cancer predisposition. However, it is not known whether the accumulation of genetic aberrations and DNA breaks in normal aging cells is caused by a primary defect in DSB repair or whether it reflects secondary changes associated with aging. A small number of studies in model organisms suggest that aging destabilizes HR control at the expense of error-free "short tract gene conversion" and in favor of error-prone "long tract gene conversion", but aging mammalian cells have not yet been studied in this regard. To address this question, we have developed a mouse strain carrying a new HR reporter that allows rapid flow cytometric analysis of short- and long-tract gene conversion in living cells. We will use this new tool to determine whether aging in mice is associated with skewing of HR in favor of error-prone pathways such as long tract gene conversion. Our Specific Aims are: Aim 1. Determine the impact of aging on the quality of HR in freshly explanted primary mouse fibroblasts. Aim 2. Determine the impact of aging on the quality of HR in freshly explanted erythroid progenitors. Success in the work described in this proposal will reveal new mechanisms underlying phenotypes associated with aging.