Trisomy 8 is the most common chromosomal abnormality found in mouse embryonic stem (ES) cell lines, while the loss of a sex chromosome and trisomy 11 have also been observed (Sugawara, et al. 2006. Comparative Medicine). ES cells exhibiting aneuploidy above 30-40% rarely produce germline transmission in the gene targeting process. This service provides a quick chromosome count and FISH screen for trisomy 8 and 11 in positive mouse ES cell clones so the use of abnormal cells for microinjection can be avoided. This simple screen greatly reduces the time and cost of generating mouse model chimeras.