Retinoblastoma tumor formation is initiated by the loss of function of both alleles of the RB-1 gene on chromosome 13. Patients with the hereditary form of retinoblastoma carry a germ line mutation at one of the two homologous gene loci in all cells and have an increased risk for nonocular tumors (mainly osteosarcoma and other mesenchymal tumors) in later life. The authors studied a 38-year-old patient with sinonasal undifferentiated carcinoma (SNUC) who had been treated for bilateral retinoblastoma by enucleation (left eye) and irradiation (right eye), respectively. Using molecular probes for the RB-1 gene and other loci on chromosome 13, the authors detected a deletion at the RB-1 locus in metastatic SNUC cells that was not present in normal tissue. These findings indicate that somatic mutations at RB-1 locus may be involved in the formation or progression of ectodermal tumors.