The interleukin-3 receptor (IL-3R) is composed of alpha and beta subunits, members of the class I cytokine receptor family. Here we describe isolation and characterization of the chromosomal gene for the mouse IL-3R alpha subunit (mIL-3R alpha). Whereas the human IL-3R alpha gene is tightly linked with the granulocyte-macrophage colony-stimulating factor receptor alpha subunit (GM-CSFR alpha) gene in the pseudoautosomal region of the X and Y chromosomes, the mIL-3R alpha gene (II3ra) is located in the proximal region of mouse chromosome 14, separated from the mouse GM-CSFR alpha gene, which is on chromosome 19. The mIL-3R alpha gene spans about 10 kb and is divided into 12 exons. All the exon-intron boundaries possess the splicing junction consensus sequences (5'GT-AG3'), and the whole genomic structure is similar to those of the previously reported class I cytokine receptor genes. There are two major transcription initiation sites that are located at 215 and 188 nucleotides upstream of the initiator codon. The promoter region is GC-rich and contains potential binding sites for GATA, Ets, c-myb,, Sp1, Ap-2, and G-C boxes, but not a typical TATA or CAAT sequence. A fusion gene containing 0.8 kb of the 5' noncoding sequence linked to the firefly luciferase gene directed the transcription in mouse mast cells but not in fibroblasts or T cells, suggesting that this promoter functions in a cell type-specific manner. Further sequential deletion of the 5' region suggests two potential regulatory regions for transcription of the mIL-3R alpha gene.