Neuroblastomas (NBs) show complex patterns of genetic abnormalities, which may include amplification of the MYCN gene, deletion of 1p, or a gain of DNA at 17q, the last being the most frequent observation in NB tumors. However, the specific genes and the molecular mechanisms responsible for development and progression of NB remain poorly understood. We investigated aberrations of DNA copy number in 25 NB cell lines using comparative genomic hybridization and identified a minimal common region of gain at 17q23. Although gain of distal 17q is the most powerful genetic predictor of adverse outcome currently available for patients with NB, thus far, no potential target genes have been reported for that region. Therefore, we defined the 17q23 amplicon in detail and determined expression levels of 15 genes located within the smallest region of overlap observed among our NB cell lines to identify the most likely target gene(s). Among them, seven (CLTC, VMP1, delta-tubulin, RPS6KB1, FLJ22087, APPBP2, and PPM1D) were consistently overexpressed through increases in regional copy number. Analysis of expression levels of those seven genes in 32 primary NB tumors revealed a significant correlation between higher expression and poorer clinical outcome only with respect to PPM1D. Moreover, down-regulation of PPM1D by transfection of an antisense oligonucleotide suppressed the growth of NB cell lines to a remarkable degree, at least partly by participating in a process leading to apoptotic cell death. Taken together, our results indicate that PPM1D is the most likely target of the 17q23 gain/amplification in NB tumors and may have an important role in the pathogenesis of this disease.