Stromal cell-derived factors 1α and 1β are small cytokines belonging to the intercrine CXC subfamily and originally isolated from a murine bone-marrow stroma cell line by the signal sequence trap method. cDNA and genomic clones of human SDF1α and SDF1β (SDF1A and SDF1B) were isolated and characterized. cDNAs of SDF1α and SDF1β encode proteins of 89 and 93 amino acids, respectively. SDF1α and SDF1β sequences are more than 92% identical to those of the human counterparts. The genomic structure of the SDF1 gene revealed that human SDF1α and SDF1β are encoded by a single gene and arise by alternative splicing. SDF1α and SDF1β are encoded by 3 and 4 exons, respectively. Ubiquitous expression of the SDF1 gene, except in blood cells, was consistent with the presence of the GC-rich sequence in the 5′-flanking region of the SDF1 gene, as is often the case in the "housekeeping" genes. Although genes encoding other members of the intercrine family are localized on chromosome 4q or 17q, the human SDF1 gene was mapped to chromosome 10q by fluorescence in situ hybridization. Strong evolutionary conservation and unique chromosomal localization of the SDF1 gene suggest that SDF1α and SDF1β may have important functions distinct from those of other members of the intercrine family.