A beta-Actin-Neo and HSC-tk sequence was inserted into the BglII site in the last exon of the Nkx2-5 gene in v6.5 ES cells. A population of properly targeted ES cells was exposed to 4Gy of radiation from a Cesium- 137 source. Deletion mutants were selected in ES cells by restoration of "tk" sensitivity. Four mouse stocks and 11 mutated ES cell lines were established. (J:86784, J:99488)