Sixteen retinoblastomas were examined with chromosome 13 polymorphic probes to determine the frequency of homozygosity for the chromosome in the tumors. Each of the tumors had two cytogenetically normal appearing No. 13 chromosomes. Nontumorous cells from the same patients were heterozygous for the various polymorphic chromosome 13 probes used. At least partial homozygosity for a single chromosome 13 was observed in 75% of the tumors. These studies confirm and extend previous studies which suggest that homozygosity or hemizygosity at RB1 occurs in the majority of retinoblastomas. We also demonstrate in an additional tumor that rapid clonal evolution from hemizygosity to homozygosity can occur in the tumor.

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Supported in part by Grants EY 06195 and EY02715 from the NIH and by the Margaret Scott Bundy Trust. This work was done in conjunction with the Clayton Foundation for Research.

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