In support of a role for CEACAM1 in regulation of lumen formation in the mammary gland, an organ that shares structural similarity with normal and malignant prostate, we have found that mammary epithelial cells lacking CEACAM1 fail to form lumina in Matrigel culture while forced expression of CEACAM1 in breast cancer cells restores lumen formation [4]. Here, CEACAM1 is linked to breast cancer.