A neomycin resistance cassette was inserted into exon 2 of the gene, which encodes the signal peptide cleavage site and the IgV domain. Monoclonal antibodies recognizing the Ig domain failed to detect any CD47 antigen on the surface of erythrocytes, bone marrow cells, or other cells including peripheral blood leukocytes, splenocytes, and fibroblasts from homozygous mutant mice. In addition, no antigen was detected in mutant erythrocyte lysates with polyclonal antisera to the NH2 or COOH-termini.