The ability of CEACAM1-4S expression in the context of the proteome of 253-NT cells to restore the tumorigenic and anchorage independent growth characteristics of the parental 253T cell line [6] provided a quantifiable, reproducible endpoint for examining the functionality of the CEACAM1-4S transmembrane domain. The idea that CEACAM1 phenotypes could be context specific came from our previous studies showing that CEACAM1-4L expression dramatically suppressed the tumorigenicity of CEACAM1 negative PC-3 human prostate carcinoma cells [13] . With continued passage, however, cells eventually reacquired a tumorigenic phenotype without losing expression of CEACAM1-4L, suggesting a strong selection for cells with proteomes that were unable to support CEACAM1 mediated tumor suppression. Context dependent effects on CEACAM1-4S phenotypes were also suggested by the ability of CEACAM1-4S to induce morphogenesis of MCF7 cells in vitro but not in vivo [21] .