Having obtained successful germline transmission of the human CEACAM1 gene and CEACAM1 protein expression in the BAC2-12 line of mice, we analyzed tissues from the transgenic mice by immunohistochemical staining to determine if expression of the human and murine genes was similar. First we analyzed heart as a negative tissue to eliminate the possibility that inappropriate expression occurred. Neither murine nor human CEACAM1 was expressed in the hearts of BAC2-12 transgenic mice, and wild type FVB mouse heart is shown as an additional control (Fig. 4A-C) . Three tissues that exhibit high expression of CEACAM1 in both mouse and human are the intestine, kidney, and liver. As seen in Fig. 4D -F, the expression of human CEACAM1 is similar to murine CEACAM1 in the small intestine in terms of distribution and intensity. As expected, both the murine and human CEACAM1 are expressed on on the luminal surface of intestinal epithelial cells. In the case of the kidney, the expression of human CEACAM1 is noticeably higher than the expression of murine CEACAM1 (compare Fig. 4H to Fig. 4G , transgenic, and Fig. 4I, wild type FVB) . This difference may be explained by the fact that mice have two Ceacam genes (Ceacam1 and Ceacam2) that are differentially expressed in the kidney [35, 37] , while humans have a single CEACAM1 gene. Thus, the expression of the human gene in the kidney may reflect the combined expression of the two genes in mouse; however, the anti-murine CEACAM1 antibody used here does not detect murine CEACAM2. The reason for the differential expression of the two mouse genes in the kidney is unexplained, but may reflect a role for the kidney in insulin clearance (see below). In the case of the liver, the staining pattern for murine and human CEACAM1 in the transgenic animals is identical (Fig. 4J-K) . The pattern shows highest expression around the portal triads with decreased expression around the portal artery. This may reflect the known function of CEACAM1 in effecting insulin clearance [39] . The observed CEACAM1 expression pattern follows the entrance of blood from the portal circulation and exit via the portal artery. The staining pattern for the murine CEACAM1 of wild type mice liver (Fig. 4L) is similar but less intense. Further quantitative studies are required to determine if there is a real difference in degree of expression of the endogenous murine Ceacam1 gene between the wild type and transgenic animals.