The gli gene was originally isolated from DNA amplified in double minutes in a glioblastoma (D259MG). Using a sensitive RNA-RNA hybridization, we tested a series of central nervous system tumors for expression of the gli gene. These included 8 glioblastoma cell lines, plus cell cultures of 5 glioblastomas, 4 anaplastic astrocytomas, 3 different ependymal tumors, a malignant meningioma, and a medulloblastoma. Two normal glial cell cultures were also examined. There was no gli expression in any of these specimens. In glioblastoma D259MG, approximately 130 molecules of gli mRNA per cell were present and the half-life of the mRNA was approximately 5 h. By reverse transcription and PCR, gli mRNA was observed in 4 cell lines and in normal human glial cells, but the level was estimated to be less than one five hundredth of that in the D259MG cell line. The results suggest that gli expression in central nervous system tumors is a rare event and mostly likely associated with amplification of the gene.