A novel tumor suppressor, PTEN/MMAC1, was recently found on chromosome 10q23 and mutations of this gene were described in about 20% of primary glioblastomas (GBM) and 60% of GBM cell lines. To define further the relevance of PTEN/MMAC1 mutations in GBM we investigated by SSCP analysis its coding sequence in 44 gliomas, including 41 GBM, and in 21 short-term cultures (15 GBM and six malignant astrocytomas). Loss of heterozygosity (LOH) at 10q23 was present in at least one marker in the vicinity of the PTEN/MMAC1 locus in 59% of the informative GBM (primary tumors and cell cultures). SSCP variant bands were found in seven primary GBM (17%) and in one short-term GBM culture and sequence analysis confirmed the presence of somatic mutations in all these cases (five missense, one splicing mutation and two small deletions). These data indicate that PTEN/MMAC1 is inactivated in a subset of GBM and suggest that the high mutation frequency previously found in GBM established cell lines reflects culture condition artifacts rather than the true mutation frequency in vivo. Other suppressors, located on chromosome 10q, may also have a critical role in glioma tumorigenesis.