Comparison of the efficiency of gene trap with different drug selection systems. (A) Design of the gene trap vectors containing the different drug resistance genes. The drug resistance genes were fused to EMCV-IRES and placed downstream of the splice acceptor (SA) of Engrailed (En)-2 gene . (B) The numbers of drug-resistant colonies in gene-trap screening were counted in each drug selection. Columns and bars represent average colony numbers and standard error means (s.e.m.) among the triplicate per 2 × 107 mES cells transfected with 100 μg of each promoter trap vector, respectively.