As part of an effort to identify informative molecular markers for genetic analysis of human pedigrees segregating for obesity, we have developed a genetic map of human 1p in the region of the OB receptor (OBR), the gene that is defective in murine diabetes (Obrdb) and rat Zucker fatty (Obrfa) mutations located on mid-chromosome 4 and chromosome 5, respectively. OBR was mapped 0.9 cR centromeric to WI-9515 and 2.2 cR telomeric of WI-7249 by radiation hybrid (RH) mapping. Ten yeast artificial chromosomes (YACs) containing OBR were identified, confirming the location of OBR centromeric to WI-9515 and telomeric to WI-7249. Additionally, five P1 artificial chromosomes (PACs) were identified that comprised a contiguous series of overlapping clones spanning the length of OBR. WI-5182 was contained within the two PACs that are 3' of OBR. Using a panel of 68 individuals from a single three-generation family and an additional nuclear family, we have mapped 18 polymorphic markers including phosphoglucomutase 1 (PGM1), which is centromeric to Obrdb / Obrfa, and D1S85, which is telomeric to Obrdb / Obrfa in the mouse and rat. The following composite map integrates these radiation hybrid, genetic, and physical maps: Centromere-@WI-7249-[OBR; WI-5182]-D1S198-[WI-9515; WI-6550; D1S2866]-D1S2825-[WI-3077; D1S2886]-[D1S515; DS1613; PGM1]-[D1S312; D1S473; D1S230; D1S246; D1S203]-D2S1643-[D1S1669; D1S1596;]UNCJ-D1S476- D1S85-D1S220-C8B-GTAT1A7. Unresolvable markers are within brackets. A comparison of gene order on mouse chromosome 4, rat chromosome 5, and human 1p indicates that between rodents and humans, there has been a rearrangement of the gene order in the region surrounding OBR.