Previous reports have demonstrated that dimerization via transmembrane domain helix-helix interactions are often mediated by GXXXG or GXXXA motifs within the transmembrane domain [30, 42] . To determine if the GXXXG motifs within the transmembrane domain of CEACAM1-4S played a role in the dimerization of CEACAM1-4S [36, 43] , the effect of G to L mutations on CEACAM1-4S interactions was analyzed by Blue-Native polyacrylamide gel electrophoresis (BN-PAGE). In BN-PAGE, Coomassie G-250 is used in place of SDS to coat proteins with a uniform negative charge without causing denaturation or disruption of protein-protein interactions [44] . As shown in Figure 10 , wild type and single G-to-L mutants of CEACAM1 demonstrated an apparent molecular mass by BN-PAGE that was approximately 100 kDa higher than the double G to L mutant, a difference approximately the size of CEACAM1-4S resolved by reducing SDS-PAGE. These data suggested that a single GXXXG motif was sufficient to mediate dimerization of CEACAM1-4S and/or interaction with another yet-to-be identified transmembrane protein, interactions that appeared to require helix-helix interactions mediated by GXXXG motifs.