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Deletions found in several types of human tumor, including carcinomas of the colorectum, breast, and lung, suggest the presence of a potential tumor suppressor gene(s) on chromosome 15. Common regions of deletion in these tumors are at 15q15 and 15q21. Here, we have analyzed loss of heterozygosity (LOH) on chromosome 15 to ascertain its potential involvement in the development and progression of transitional cell carcinoma (TCC) of the bladder. A panel of 26 polymorphic markers, spanning 15q12-15q22, were used to map regions of LOH in 51 TCCs. LOH was found for at least one marker in the region 15q14-15q15.3 in 20 of 51 (39%) tumors. Deletion mapping defined two minimum regions of deletion: a distal region between the markers D15S514 and D15S537 at 15q15.1-15q15.3 (estimated as 3 Mb) and a more proximal region between the markers D15S971 and D15S1042 at 15q14 (estimated as 1.1 Mb). Analysis of a panel of 33 bladder tumor cell lines revealed regions of contiguous homozygosity for markers in 15q15, indicating likely LOH. Fluorescence in situ hybridization analysis demonstrated that mitotic recombination is the predicted mechanism of LOH in two of these. These regions of LOH on 15q may contain tumor suppressor genes the loss or inactivation of which is associated with TCC development. The DNA repair gene RAD51 at 15q15.1 represents a candidate 15q tumor suppressor gene. Expression analysis of rad51 protein in tumor cell lines revealed variable levels of expression but no significant loss of expression in cell lines with likely 15q LOH.