The insulin-like growth factor binding proteins (IGFBPs) constitute a structurally related protein family with six known members. We have mapped and regionally localized the genes coding for human IGFBP1, 2, 3, and 4, by in situ hybridization and somatic cell hybrid analysis. The IGFBP2 gene maps to chromosomal region 2q33-q34 whereas the genes for IGFBP1 and 3 are localized in a tail-to-tail fashion on chromosome 7 region p14-p12. The IGFBP4 gene is located in the chromosomal region 17q12-21.1. Structural characterization of the genes coding for IGFBP-1, 2, 3, and 5 showed that the translated parts are divided into four exons. The exons are similar both in size and sequence in all studied genes.