Owing to their importance in normal cell division, DNA damage checkpoint and repair genes are often required for the earliest stages of embryonic development. For example, conventional deletion ofATR(1 ),Chk1(2 ),Mad2(3 ),NBS(4 ),Rad50(5 ),BRCA1(6 ),BRCA2(7 ), orRad51(8 ) leads to developmental arrest prior to gastrulation. While prior to arrest the number of cells extant in these embryos is low, procedures allowing rudimentary analysis of cell cycle checkpoints and genome integrity have been developed through culturing blastocysts in vitro (1 ). These procedures provide a small number of proliferating cells that can be analyzed for cell cycle progression, G2/M phase checkpoint responses, and gross chromosome abnormalities by mitotic spread preparation. Experiments such as these may help determine the essential functions of these genes in cell proliferation and early embryonic development. It is interesting to note that recently developed methods to introduce single-copy transgenes into one-cell zygotes via lentiviruses (9 ) may provide a means to generate Cre/lox-conditional cell lines from these conventional knockouts.
