Zhou et al [48] showed that the both the N-terminal domain and A3 mediated the interaction between CEA molecues on apposing cell surfaces; both domains are present within the CEA splice variant. Thus, we tested the hypothesis that co-expression of the CEA splice variant and full-length CEA proteins may, through heterophilic interactions, result in the MEDI-565 binding epitope being masked in the full-length form and subsequently prevent or reduce MEDI-565 binding. Co-expression of the CEA splice variant with full-length CEA on the same cells did not significantly affect the apparent binding affinity of MEDI-565 to full-length CEA (CHO FL CEA, apparent K D = 5.060.15 nM; CHO FL+SV CEA, apparent K D = 5.663.0 nM; p = 0.86). These results were expected since homophilic interactions between fulllength CEA proteins also do not appear to prevent the binding of MEDI-565 [17, 18] or other CEA-specific BiTE antibodies [16] .