Transgenic mice harboring direct repeat substrates reveal key underlying causes of homologous recombination in vivo.

Homology directed repair is a critical process for maintaining the genome of mammalian cells. While considered to be relatively error free, homologous recombination (HR) can lead to insertions, deletions, translocations, and loss of heterozygosity. Furthermore, it is known that conditions that cause HR events are often carcinogenic, and that HR modulates susceptibility to cancer chemotherapeutics, making HR relevant to both cancer etiology and treatment. To learn about HR in vivo, several laboratories have developed mouse models that harbor direct repeat substrates for which HR yields a phenotype that can be visualized by either a change in pigmentation or by expression of a fluorescent protein. An exciting aspect of this work is that it is possible to detect recombinant cells within intact tissues and thus learn about how physiological changes, genes, and exposures modulate HR susceptibility in vivo, as well as which cell types are most susceptible to HR, and the extent to which recombinant cells have undergone clonal expansion. Here, we review progress in studying HR in vivo for four different mouse strains that harbor direct repeat substrates for HR detection, namely the p