The goal of this project was to determine whether the originating strain of mouse embryonic stem (ES) cells affects the maintenance of their pluripotency under uniform culture conditions. ES cells from two strains of mice, E14 and C2J, were tested. Both ES cell lines were cultured in KOSR + 2i medium and then injected into C57BL/6J blastocysts. Our results demonstrate that this medium could support both E14 and C2J ES cells to keep their pluripotency, though E14 ES cells were found to have a higher chimeric rate than C2J ES cells. However, analysis by backcrossing revealed that C2J and E14 ES cells have the same ability for germline transmission. Our results demonstrate that ES cells derived from E14 and C2J cells have the same capacity for germline transmission when injected into C57BL/6J blastocysts; however, due to the limitation of mixed genetic background between E14 cells and host C57BL/6J embryos, C2J ES cells are preferable to E14 ES cells for use in gene-targeting and should become the cell line of choice for the generation of genetically engineered mutant mouse lines.