Although the CEA splice variant was infrequently found in pancreatic tumors, it was found at a high frequency in colorectal (98%) and gastroesophageal (50%) cancers, and to a lesser degree in lung (30%) and breast (12%) tumors. Due to its concordant expression with full length CEA in human tumors, we sought to understand the binding of MEDI-565 to the CEA splice variant and the role that this CEA isoform might play in targeting CEApositive tumors. The amino acids in full-length CEA important for MEDI-565 binding were found to be largely absent in the CEA splice variant, except amino acids I 408 and N 410 in segment C ( Figure 2 ). This observation suggested that binding of MEDI-565 to the CEA splice variant was unlikely to occur. To test this hypothesis, CHO cells were infected with lentivirus constructs directing the expression of full-length CEA (CHO FL CEA) or the CEA splice variant (CHO SV CEA), or both concurrently after sequential infection of cells with the CEA splice variant then the full-length CEA (CHO FL+SV CEA); full-length CEA and CEA splice variant protein expression were verified by western blotting ( Figure S1 ). As anticipated, MEDI-565 bound to cells expressing full-length CEA but not to cells expressing the CEA splice variant ( Figure 6A ). We note that a higher level of MEDI-565 binding was observed for cells expressing both the CEA splice variant and full length CEA together relative to cells expressing only full length CEA. This is likely due to different levels of expression efficiency in each independently infected cell line, although effects of the CEA splice variant protein on full length CEA protein levels or epitope accessibility in cells cannot be ruled out.