WW6 - AN EMBRYONIC STEM-CELL LINE WITH AN INERT GENETIC-MARKER THAT CAN BE TRACED IN CHIMERAS

Mutant mice produced by gene targeting in embryonic stem (ES) cells of ten have a complex or embryonic lethal phenotype. In these cases, it w ould be helpful to identify tissues and fell types first affected in m utant embryos by following the contribution to chimeras of ES cells ho mozygous for the mutant allele. Although a number of strategies for fo llowing ES cell development in vivo have been reported, each has limit ations that preclude its general application. In this paper, we descri be ES cell lines that can be tracked to every nucleated cell type in c himeras at all developmental stages. These lines were derived from bla stocysts of mice that carry an 11-Mb beta-globin transgene on chromoso me 3. The transgene is readily detected by DNA in situ hybridization, providing an inert, nuclear-localized marker whose presence is not aff ected by transcriptional or translational controls. The ''WW'' series of ES lines possess the essential features of previously described ES lines, including giving rise to a preponderance of male chimeras, all of which have to date exhibited germ-line transmission. In addition, c lones selected for single or double targeting events form strong chime ras, demonstrating the feasibility of using WW6 cells to identify phen otypes associated with the creation of a null mutant.