In general, tumors formed by cells expressing the single and double tyrosine mutants were much larger than those generated by cells transfected with wild type CEACAM1-4S or empty vector, a trend consistent with the greater rate of proliferation shown in vitro by tyrosine mutants. When viewed as a whole, these findings show that the phenotypes manifested in vitro by cells expressing transmembrane domain mutants are recapitulated in vivo, suggesting these changes are intrinsic to the transmembrane domain and are not the result of extrinsic factors e.g., microenvironment, that change the phenotypic effects of CEA-CAM1-4S by altering its proteomic context.