Previous karyotyping showed an unbalanced translocation between chromosome 3p11.2-p13 and chromosome 5q22 leading to duplication of chromosome 5q22-qter region in nonpapillary renal cell carcinomas. In order to determine the breakpoint at chromosome 5q22 at the molecular level, we have investigated 50 sporadic nonpapillary renal cell carcinomas from consecutive nephrectomies and 24 renal cell carcinomas obtained from two patients with von Hippel ± Lindau disease. We used seven DNA markers mapped to and around the APC and MCC genes to detect allelic imbalance. We observed a duplication of chromosome 5q sequences in 11 of 23 informative sporadic tumours and in 18 of 24 hereditary tumours. We determined a breakpoint cluster between the APC and MCC genes at chromosome 5q22. In addition we have found a partial duplication of the smallest overlapping region of about 1.5 Mb sequences including the MCC gene in seven tumours without visible alteration of chromosome 5q in the karyotype. We suggest that this DNA segment harbours a gene or gene cluster, the altered dosage of which is important for the growth of nonpapillary renal cell carcinomas.