Analysis of the amino acid sequence of the CEACAM1-4S transmembrane domain revealed the presence of four GXXXG sequences (Table 1 and 2), a motif known to drive high affinity helix-helix interactions that stabilize the dimerization/oligomerization of many well characterized proteins such as glycophorin A, epidermal growth factor receptor and the G protein-coupled afactor receptor of budding yeast [30, 42, 45] . For the latter protein, disruption of the GXXXG motif not only impaired oligomerization but also disrupted signaling [30, 46] . Previous investigations have shown that both the long and short isoforms of CEACAM1 form cis-dimers [36, 43] , an interaction that we hypothesized should be stabilized by GXXXG mediated helix-helix associations. To test this idea, we introduced glycine-to-leucine mutations that disrupted one or more transmembrane domain GXXXG motifs. FACS, fluorescent confocal microscopic and immunoblot analysis of cells stably transfected with the glycine-to-leucine mutants confirmed that the mutant proteins were expressed on the cell surface at the same level and the same size as wild type CEACAM1-4S. However, after 3 weeks in soft agar, cells expressing single glycine mutants showed significantly lower rates of proliferation and significantly smaller colonies when compared to cells transfected with wild type CEACAM1-4S, suggesting that a single glycine-to-leucine mutation had compromised the ability of CEACAM1-4S to induce anchorage independent growth. From these results, we concluded that the smaller size of colonies produced by glycine mutants resulted at least in part from a decrease in the rate of proliferation.