The best studied oncogenic mechanisms are inactivating defects in both alleles of tumor suppressor genes and activating mutations in oncogenes. Chromosomal gains and losses are frequent in human tumors, but for many regions, like 1p36 and 17q in neuroblastoma, no mutated tumor suppressor genes or oncogenes were identified. Amplification of N-myc in neuroblastoma is strongly correlated with loss of 1p36 and gain of 17q. Here we report that N-myc down-regulates the mRNA expression of many genes with a role in cell architecture. One of them is the 1p36 gene Cdc42. Restoring the Cdc42 expression in neuroblastoma cells strongly induced differentiation. N-myc also inhibited Cdc42 functioning at the protein level. This was mediated by nm23-H1 and nm23-H2, which are located in the amplified 17q region. Nm23-H1 and nm23-H2 are strongly up-regulated downstream targets of N-myc. Nm23-H1 was shown to bind Cdc42 and prevented the induction of differentiation. Overexpression of Nm23 due to gain of 17q and induction by N-myc combined with weak expression of Cdc42 due to loss of 1p36 and down-regulation by N-myc can thus block differentiation. Although this marks Cdc42 as a candidate tumor suppressor gene, no mutations were found. Further silencing of Cdc42 by small interfering RNA induced massive apoptosis, indicating that tumor cell survival requires a minimal Cdc42 activity. Three regions of chromosomal gain and loss thus affect genes functioning in one pathway in neuroblastoma. They converge to bring the pathway out of balance and prevent Cdc42 mediated differentiation.