Exons 16 and 17 of the endogenous gene, which encode the cytoplasmic domain, were replaced by an Epor cDNA fragment. The resultant allele encodes a chimeric receptor with the endogenous extracytoplasmic (ligand binding) and transmembrane regions adjoined to the Epor cytoplasmic (signaling) region. Flow cytometric and ligand binding assays indicated that the chimeric receptor had normal binding affinity and was expressed on the surface of homozygous mutant neutrophils at a level approximately 80% of normal expression.