Antibody binding to CEACAM1 constructs. NA1B-Fc proteins were dotted at 0.2 mg ml -1 on to nitrocellulose and non-specific binding sites blocked using 3% (w/v) BSA in Dulbecco's PBS containing 0.05% Tween-20 (PBST) for 1 h at room temperature. Receptor was detected using the following antibodies, rabbit polyclonal AO115, rat monoclonal YTH71.3 both directed against the N-domain and mouse monoclonal Kat4c which recognizes the A and B domains (Jones et al., 1995) . Bound antibody was subsequently detected using an appropriate secondary antibody conjugated to alkaline phosphatase and developed using nitroblue tetrazolium and 5-bromo-4-chloro-3-indolylphosphate.