We have isolated and characterized the gene coding for the mouse Fc receptor that is termed Fc gamma RIIa. The gene contains five exons and spans approximately 9 kilobases. Unlike most members of the immunoglobulin gene superfamily, this gene utilizes multiple exons to encode its leader peptide. The first exon encodes the hydrophobic region of the signal sequence; the second exon, which contains only 21 base pairs, encodes a segment of the signal peptidase recognition site; and the beginning of the third exon encodes the predicted site of peptidase cleavage. The third and fourth exons each code for immunoglobulin-like extracellular domains. The fifth exon encodes the hydrophobic transmembrane domain and the cytoplasmic tail. Partial characterization of the Fc gamma RIIb gene indicates that it also contains multiple leader exons, including a 21-base-pair exon and two exons coding for homologous immunoglobulin-like extracellular domains. However, the Fc gamma RIIb gene uses four exons to encode its intracytoplasmic region. Analysis using contour-clamped homogeneous electric field (CHEF) gels indicates that the Fc gamma RIIa and Fc gamma RIIb genes are linked within 160 kilobases on mouse chromosome 1.