The protein encoded in a novel human oncogene isolated by transfection of Kaposi's sarcoma DNA is a growth factor with significant homology to basic and acidic FGFs. The genomic structure of this oncogene (designated K-FGF), as originally isolated, carried DNA rearrangements upstream and downstream of the coding region. The normally discontinuous sequence upstream of the K-FGF coding region derived from the 3' end of the c-fms gene and thus originated from human chromosome 5. In order to determine the normal chromosomal location of the K-FGF gene and of the DNA sequences adjacent to its 3' end, we have correlated the presence of these sequences with retention of specific human chromosome regions in rodent-human somatic cell hybrids. These experiments mapped the K-FGF gene to human chromosome region 11q13----11q23, and in situ hybridization localized it more precisely to region 11q13 near int-2, which also belongs to the FGF family. The sequence downstream of the gene in transfectants and discontinuous with K-FGF in normal human DNA derives from chromosome region 12p12----12q13, possibly near the int-1 locus.