Chimeric soluble receptor proteins containing the appropriate amino acid substitutions were prepared as previously described by transient transfection of COS cells (Teixeira et al., 1994; Virji et al., 1999) . The CEACAM1-Q89A-Fc used in overlay experiments was kindly donated by Dr S. Watt (Virji et al., 1999; Watt et al., 2001) .