In keeping with their limited capacity for anchorage independent growth, cells transfected with empty vector were poorly tumorigenic relative to cells expressing the wild type CEACAM1-4S. As predicted from their growth in soft agar, cells expressing the G424L mutant formed tumors considerably smaller than those derived from cells expressing wild type CEACAM1-4S. However, the similarity in the size of tumors generated by cells expressing the wild type, double glycine and double tyrosine mutants was at odds with in vitro assays where cells expressing wild type CEACAM1-4S showed a significantly greater or lesser capacity, respectively, for anchorage independent growth than cells expressing double glycine or tyrosine mutants. This discrepancy is reminiscent of the differential in glandular morphogenesis exhibited in vitro and in vivo by MCF7 mammary carcinoma cells [21] and thus could reflect differential effects of the subcutaneous microenvironment.