The decrease in soft agar growth produced by glycine mutations and the increase by tyrosine mutations relative to the wild type CEACAM1-4S would classify these, respectively, as loss-of-function and gain-of-function mutations [47, 48, 49] . Gain-of-function mutations are usually dominant, a characteristic apparent for the glycine/tyrosine mutants of CEACAM1-4S where the growth enhanced phenotype of tyrosine mutants was dominant over the growth inhibited phenotype resulting from glycine mutations. Based on the known functions of GXXXG motifs, it seems likely that the loss-of-function caused by glycine mutations is related to changes in helix-helix interactions that destabilize rather than prevent the formation of cis-dimers [28, 50, 51] , a possibility consistent with the fact that single G424L or G432L mutations failed to disrupt dimerization ( Figure 10 ) but did cause a decrease in anchorage independent growth. Also noteworthy is the marked decrease in cisdimerization ( Figure 10 ) when mutations were introduced at both G424 and G432, a result that suggested G424-AIA-G428 was a functional GXXXG motif capable of directing cis-dimerization in the absence of the two motifs centered on G432 and necessary to sustain the level of growth produced by wild type CEACAM1-4S. Although this conclusion is seemingly at odds with the 9 prediction algorithms that placed G424 outside the transmembrane domain (Table 1) , we suggest that a combination of structural and functional data provides a more accurate location of the transmembrane domain, one that incorporates the GAIAG motif at the N-terminus.