Three BAC clones containing the human CEACAM1 gene were microinjected into the pronuclei of fertilized oocytes of FVB N/J mice and founder transgenics were generated with BAC2 and BAC5 clones. BAC2Clone (214 kb) covered a large region proximal to human CEACAM1 on chromosome 19, and two other BAC clones (BAC3 and BAC5; 138, and 211 kb, respectively) covered shorter regions flanking the human CEA-CAM1 gene. Although litters were obtained from mice fertilized with all three microinjections, only those from BAC2 and BAC5 transmitted the germline gene to their offspring (Fig. 1A and data not shown), and only those from BAC2 exhibited expression of the huCEACAM1 protein in their feces (Fig. 1B) . Since CEACAM1 is heavily expressed in the GI tract, from the liver throughout the small and large intestine, and is shed into the lumen of the gut like CEA [34] , we used its detection in the feces as an early diagnostic test for protein expression. As can be seen in Fig. 1B , both murine and human CEACAM1 were detected in the feces of the founders. Only the offspring from founder 12 (BAC2) expressed the transgene. As expected, roughly 50% of the pups were positive for human CEACAM1 by both PCR analysis of tail samples (Fig. 1C) and western blot analysis of fecal samples (Fig. 1D ) of the F1 generation from founder 12 (BAC2). Further breeding of the BAC2-12 transgenic mice demonstrated transmission of an expressed CEACAM1 transgene from the F1 to F2 generation at the expected (,0.5) frequency for heterozygotes. No attempt was made to breed the BAC2-12 line to homozygosity at this stage of the study. Expression of the human CEACAM1 transgene was not detected in any of the BAC5 founders or their F1 offspring. Given the small number of litters obtained from each microinjection, it was not feasible for us to evaluate the success rate in terms of stable integration and expression of the various BAC clones.