The effect of transmembrane domain mutations on anchorage independent growth was determined from changes in the average areas of soft agar colonies ( Figure 5 and 6) . When 253T-NT cells expressing wild type ( Figure 1C) , single or double tyrosine mutants of CEACAM1-4S were grown in soft agar, sublines expressing tyrosine mutants (Figure 5 D, E, F and Figure 6 ) showed a 3.5-fold increase in colony size relative to cells transfected with the wild type protein ( Figure 1C and Figure 6 ). In contrast, 253T-NT cells expressing CEACAM1-4S with GXXXG motifs disrupted by G to L mutations ( Figure 5A , B, C and Figure 6 ) either did not grow or formed colonies that for the double glycine mutant ( Figure 5C and 6), were less than half the size of those formed by cells expressing the wild type protein ( Figure 1C and Figure 6 ). Moreover, the rapid growth phenotype conferred by tyrosine to valine mutants appeared to be dominant over the growth suppressed phenotype displayed by glycine to leucine mutants since all of the sublines with both glycine to leucine and tyrosine to valine mutations displayed enhanced anchorage independent growth ( Figure 5G , H, I and Figure 6 ).