A gene trap construct was used to transfect mouse ES cells. 36 ES colonies trapped by one copy of neo gene were obtained. The β-galactosidase activity was detectable in 14 ES colonies. ES cells from 3 trapped lines were introduced into blastocysts by microinjection. Two chimeric mice lines were generated. One trapped mutation went through the germline. Genomic sequences adjacent to integration sites of the constructs were isolated by plasmid rescue. The results of sequencing suggested that the trapped gene is possibly a novel gene. The expression pattern of this gene shown by β-galactosidase expression was restricted in abdomen and lib bud of E10.5 mouse embryo.