Loss of heterozygosity at 10q26 was mapped using microsatellite markers in 20 osteosarcomas. A four-megabase region centered on marker D10S587 was affected by allelic loss in 60 percent of osteosarcomas. The most frequently lost marker was D10S1723. Around 15 known genes are found in this region. The gene immediately adjacent to D10S1723 encodes BUB3, an element of the spindle assembly mitotic checkpoint. Loss of BUB3 function could contribute to chromosomal instability. The fibroblast growth factor receptor 2 (FGFR2) gene is located 2 Mb from the BUB3 gene and has the potential for a role in cancer. Inherited mutations of the FGFR2 gene result in skeletal dysplasias. FGFR2 alterations have also been implicated in gastric cancer. Human genome project data were used to design primers for amplifying FGFR2 in 18 genomic segments and BUB3 in 7 genomic segments. In each case, the segments encompassed coding exons and flanking intron sequences. The primers were used to search for mutations by polymerase chain reaction-single-strand conformation polymorphism (PCR-SSCP). Several shifted bands were detected in the BUB3 exon 3 fragment. Sequencing resolved the BUB3 exon 3 fragment shifts into polymorphisms in intron 2. No mutations of BUB3 or FGFR2 were detected. It remains possible that BUB3 or FGFR2 hemizygosity alone contributes to osteosarcoma, or that one of the genes is cryptically inactivated by a higher-order modification or mutation outside the coding region. There may also be a yet undiscovered tumor suppressor gene in this region.