The gene encoding murine Chst4 (in BAC clone RP23-362O14) was homologously recombined with a PCR fragment containing the cre recombinase cDNA and frt-flanked kanamycin-resistance gene flanked by 50 bp sequences of Chst4. The linearized recombinant BAC was injected into fertilized mouse zygotes which were implanted into pseudopregnant foster mothers. Four founder lines were generated and one was used for further studies.