We have previously reported on the amplification and overexpression of the MDM2 proto-oncogene in a subset of malignant gliomas without TP53 mutation (G. Reifenberger et al, Cancer Res., 53: 2736-2739, 1993). Here, we show that the MDM4 (MDMX) gene located on 1q32 is a further target for amplification in malignant gliomas. MDM4 codes for a Mdm2-related protein that can bind to p53 and inhibits p53-mediated transcriptional transactivation. We investigated a series of 208 gliomas (106 glioblastomas, 46 anaplastic gliomas, and 56 low-grade gliomas) and identified 5 tumors (4 glioblastomas and 1 anaplastic oligodendroglioma) with MDM4 amplification and overexpression. Several other genes from 1q32 were found to be coamplified with MDM4, such as GAC1 in five tumors, REN in four tumors, and RBBP5 in three tumors. Additional analyses revealed that the malignant gliomas with MDM4 amplification and overexpression carried neither mutations in conserved regions of the TP53 gene nor amplification of the MDM2 gene. Taken together, our data indicate that amplification and overexpression of MDM4 is a novel molecular mechanism by which a small fraction of human malignant gliomas escapes p53-dependent growth control.