The present invention provides a method of achieving very high targeting efficiency by utilizing targeting vectors that utilize promoter-less selection cassettes and which are engineered to targeted into transcriptionally active loci. In particular, the invention provides a method for targeting promoter-less selection cassettes into transcriptionally active loci in stem cells or other eukaryotic cells with much greater efficiency than previously observed with other methods, thus reducing the number of drug-resistant clones to be screened or eliminating the need to screen for targeted cells altogether. The invention also encompasses the DNA targeting vectors, the targeted cells, as well as non-human organisms, especially mice, created from the targeted cells.