The diphtheria toxin A-chain gene was used in a positive-negative selection gene targeting vector to alter the CD4 gene which is transcriptionally silent in mouse embryonic stem cells. Expression of the toxin gene was driven by a constitutively active enhancer, yet the targeting construct exhibited only minimal transient toxicity while enriching for targeted clones 9- to 29-fold. Germline transmission of the stem cell-derived genome was obtained. These data suggest the usefulness of this diphtheria toxin A-chain cassette in replacement-type positive-negative selection vectors. Its potential for novel applications, particularly in the enrichment for 'hit-and-run' insertion-type vectors, is discussed.