Many retinoblastomas (Rbs) show genomic alterations in addition to mutational loss of both normal RB1 alleles. The most frequent of these changes are gains on chromosomes 1q and 6p and losses on 16q. To identify the genes targeted by gains on chromosome 1q, we used quantitative-multiplex PCR to determine DNA copy number changes in 76 primary tumors and 6 Rb cell lines. In addition, in 21 of these tumors, gene expression was analyzed by cDNA microarray hybridization. Increased copy numbers of loci on chromosome 1q were present in 34 (45%) primary tumors and in all 6 cell lines. Two regions of gain emerged, one in 1q32 and another in 1q21. Tumors with 1q gains showed higher RNA expression of several genes in these 2 regions. The clinical manifestation of tumors with and without gains was similar with regard to many aspects, including size, necrosis and calcification. However, the distribution of age at diagnosis was remarkably distinct, with earlier diagnosis in tumors without gains. This suggests that these tumors either are initiated earlier or grow faster than tumors with gains. This association with clinical manifestation indicates that gains on 1q are significant for the biology of Rb. The genes on 1q with copy number gains and overexpression are candidates that need to be tested for their individual contribution to the progression of Rb. ' 2005 Wiley-Liss, Inc.