The consistent luminal expression of CEACAM1 on epithelial cells supports the hypothesis that CEACAM1 plays a role in lumen formation in several tissues [5, 6, 7] . This role has been documented in mammary epithelial cells grown in either a 3D culture [5, 6, 7] or in humanized mammary fat pads in NOD/SCID mice [41, 42] . CEACAM1 is also expressed on the luminal surface of prostate glands [43] , and like its expression in the mammary gland, is down-regulated in cancer [44] . As a first step in using these transgenic animals as a model for mammary and prostate cancer we examined its expression in these tissues. In the case of the mammary gland, it was necessary to study pregnant females, since the mammary gland is minimally developed in non-pregnant females. As shown in Fig. 6A -C, there was strong luminal expression of both murine and human CEACAM1 in the mammary glands of pregnant transgenic mice. There was also evidence for cytoplasmic expression, perhaps indicating active transport of CEACAM1 from the site of synthesis to the lumen. Similar results were seen for the prostate glands of male human CEACAM1 transgenic mice (Fig. 6 D-F) . These results suggest that these mice may be an appropriate model for studying the role of human CEACAM1 in tumorigenesis within these organs.