A method was developed to clone, without the use of specific functional assays, complementary DNAs (cDNAs) that carry specific amino-terminal signal sequences, such as those encoding intercellular signal-transducing molecules and receptors. The vector used in this system directed the cell surface expression of interleukin-2 receptor fusion proteins when inserts with signal sequences were cloned in-frame with the correct orientation. An expression cDNA library was constructed from a bone marrow stromal cell line, which contained 5' portion-enriched cDNAs (the average size was 400 base pairs). Two cDNAs that encoded putative cytokine molecules, stromal cell-derived factor-1 alpha (SDF-1 alpha) and SDF-1 beta, which belong to the intercrine-macrophage inflammatory protein superfamily, were cloned.