ObjectiveTo explore the presence of common genetic alterations in retinoblastoma and to localize the altered genomic regions.MethodsGenetic instability of chromosome 19, 20, 21, 22 and X of 15 microdissected retinoblastoma tumors were analyzed by the loss of heterozygosity (LOH) and microsatellite instability (MSI).ResultsAmong the 15 patients with retinoblastoma, genome instability ［LOH and(or) MSI］ at one or more loci on the 5 chromosomes in 10 (67%), in which the loss of a single allele was more frequent in chromosomes 19 (33%) and 20 (27%) than in the other 3 chromosomes. Highfrequency LOH between D19S902 and D19S571 suggested gene loci in the 19q13 region might be associated with tumor development in retina. According to the result of MSI, MSI occured at least in one subset of retinoblastoma.ConclusionsOur results provide first evidence of LOH in chromosomes 19 and 20 in retinoblastoma and further support the presence of genome instability in retinoblastoma that may play an important role in the tumorigenesis or progression of retinoblastoma.(Chin J Ocul Fundus Dis, 2005,21:28-31)