Both murine Ceacam1 and human CEACAM1 genes produce multiple mRNA splice isoforms. Murine Ceacam1 generates mRNAs having mainly 4 or 2 ectodomains with long or short cytoplasmic domains. Indeed, the expected major splice isoforms of murine Ceacam1 were found in the liver, intestine, and kidney of both control and transgenic mice with a predominance of the -2S isoform. The human CEACAM1 gene, in contrast, generates mRNAs with different splice isoforms, namely -4L/S and -3L/S. The expected human splice isoforms were observed in the BAC2-12 transgenic line, with a predominance of -4S in the liver and -4L in the intestine and kidney. Given the differences in murine and human splice isoforms observed in the transgenic animals, we Figure 9 . Expression levels of murine and human CEACAM1 on spleen B and T cells of BAC2-12 transgenic and wild type mice. Freshly isolated spleen cells (A) and Spleen cells treated with 1 mg/mL anti-CD3 and 2 mg/mL anti-CD28 mAb for three days (B) from TG or WT mice were stained with either the anti-murine CEACAM1-PE or anti-human CEACAM1-PE and co-stained with anti-CD3-FITC and anti-B220-APC. Results shown are representative of three independent experiments with similar results. doi:10.1371/journal.pone.0010067.g009