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 lp in the region of the OB receptor (OBR), the gene that is defective in murine diabetes [Obr db) and rat Zucker fatty [Obr re) 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 PI 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 [PGMI), which is centromeric to Obrdb/Ob~, and DIS~5, which is telomeric to ObrdblObr ~a in the mouse and rat. The following composite map integrates these radiation hybrid, genetic, and physical maps: