We describe an oligonucleotide array system for CGH analysis with an improvement in resolution to ~75 nucleotides without any target or signal amplification,capable of detecting two or more extra DNA copies.Using only a few micrograms of genomic DNA we found or confirmed several low-copy number amplifications in cancer cell lines BT474 and Colo320.Furthermore we demonstrate that the level of gene expression correlates with the level of gene amplification and that the same array can be used for both analyses.This advance significantly simplifies the preparation of probe DNA for array production,avoids the need to block repetitive sequences,and makes possible the genomic and expression analysis of any genome for which sequence data are available.