The chromosomal insertion point for the BAC clone is random in transgenic animals produced by pronuclear microinjection. Using human specific probes for CEACAM1, we were able to demonstrate that the human gene is on chromosome 19 of human cells ( Fig. 2A) , as expected, and the BAC2 clone containing the human CEACAM1 gene was on chromosome 11 of the transgenic mice (Fig. 2C-D) . Although we did not confirm the location of the endogenous murine Ceacam1 gene, it has been reported to be on chromosome 7 [35, 36] . Thus, the murine Ceacam1 and human CEACAM1 genes are on different chromosomes of the transgenic mice. Since mRNA from the human CEACAM1 can be alternatively spliced, it was also important to determine if the expected splice forms were present. For this analysis, RNA was isolated from BAC2-12 transgenic mouse liver, kidney and small intestine and analyzed by PCR using specific primers for the major splice forms expected in human CEACAM1. As seen in Fig. 3A , transgenic mouse liver expressed human CEACAM1-4S and -3S isoforms, while transgenic kidney and small intestine expressed the -4L and -3L isoforms. Analysis of the endogenous expression of murine CEACAM1 isoforms revealed the -4S and -2S isoforms in the liver and intestine, while the kidney expressed mainly the -2S isoform (Fig. 3B) . Since mice express a highly homologous second allele, namely CEACAM2, mainly expressed in the testes and kidney, we cannot be sure if the kidney PCR products are from murine CEACAM1 or CEACAM2. The differences in tissue specific CEACAM1 isoform expression between mouse and man has been previously described [37, 38] .