Production of mutants was carried out by site-directed mutagenesis of the pIG construct containing the DNA encoding the CEACAM1 NA1B domains described previously (Watt et al., 1994) . The oligonucleotide primers used to create amino acid substitutions at positions 91, 34 and 89 of the N-domain are shown in Table 1 . Some primer sequences have been published previously (Watt et al., 1994) . For introducing mutations, CEACAM1 was amplified by polymerase chain reaction from the pIG-NA1B construct using either the common forward primer and a reverse primer containing the desired mutation, or a complementary forward primer containing the mutation and a common reverse primer. CEACAM1 with the appropriate mutation was amplified using the common forward and common reverse primers. The gene was then cloned into pIG using the restriction sites HindIII and EcoRI.