Human epithelial tumors need to accumulate multiple genetic alterations to form invasive carcinomas. These genetic alterations are related with growth factor receptors, cell signalling, the cell cycle and cell invasiveness. Importantly, cells need to avoid senescence and become immortalized for this process. Recently, five genes: RPS6KA6, HDAC4, KIAA0828, TCP1 and Tip60, which modulate p53-dependent function and avoid senescence were identified in a large-scale RNA interference screen. Twenty colon, 20 prostate and 20 lung carcinomas were studied to investigate whether these genes might be related with human tumors. RNA was extracted from both normal and tumor tissue from each patient. Real-time RT-PCR was performed using TaqMan probes corresponding to the RPS6KA6, HDAC4, KIAA0828, TCP1, Tip60 and p53 genes. In colon carcinomas, the RPS6KA6, HDAC4, KIAA0828 and Tip60 genes were downregulated in tumor tissue as compared with normal tissue (P < 0.001 for all genes). In lung carcinomas, HDAC4, KIAA0820 and Tip60 were downregulated (P < 0.01, P < 0.001 and P < 0.001 respectively). Whereas no significant differences were observed in prostate carcinomas, striking downregulation of the RPS6KA6 and KIAA0828 genes was observed in colon carcinomas and KIAA0828 in a subset of lung carcinomas. mRNA expression of these genes may control p53 function as well as the ras-MAPK pathway, methylation and transcriptional cellular programs. These results could unravel a novel set of regulatory suppressor genes involved in human colon and lung tumors.