Medulloblastoma, a primitive neuroectodermal tumor that arises in the posterior fossa, accounts for 20% of childhood CNS tumors. Histological and molecular studies suggest that this is a heterogenous tumor. The nevoid basal cell carcinoma syndrome (NBCCS) is an AD disorder characterized by medulloblastoma and basal cell carcinoma of the skin (BCCs). The NBCCS gene was mapped to 9q22-31 and function as a tumor suppressor. The NBCCS gene is the human homologue of a drosophila developmental gene, patched. Patched is a component of the hedgehog signalling pathway, important in the early patterning of the brain and limbs in vertebrates. Patched encodes a transmembrane protein that functions as a hedgehog receptor, specifically repressing its function. Smoothened, a second transmembrane protein, is a positive regulator of the hedgehog signal. Activation of the pathway results in upregulation of several genes including members of the Wnt family, TGF-β family and patched itself. Only a small subset of sporadic medulloblastomas (10-20%) show LOH for markers on chromosome 9 as compared to sporadic BCCs (70% LOH). Activation of the hedgehog pathway appears to be a necessary step for BCC formation as inactivating mutations of patched or activating mutations of smoothened have been found in most sporadic BCCs. We examined DNA samples from 23 sporadic medulloblastomas and 3 cell lines (ATCC: D283 Med, Duoy, D341) to determine the role of the hedgehog pathway in sporadic medulloblastomas. The samples were screened by SSCP analysis for mutations in patched and two mutations were found in two tumors. A one base pair deletion in exon 21 which results in a premature stop codon was found in a tumor with LOH, confirming inactivation of patched. A three bp deletion -22 to 25 base pair 3' to exon 11 was found in a second tumor without LOH. This may represent a rare polymorphism but was not found in 20 normal individuals. No patched mutations were found in the three cell lines by SSCP or direct sequencing. The tumors and cell lines were also screened for gain of function mutations in sonic hedgehog (His133 to Tyr) and in smoothened(Trp535 to Leu) and no mutations were found. Expression studies on the three cell lines by Northern blot and RT-PCR showed no upregulation of patched or Gli-1 as seen in BCCs. Activation of the hedgehog pathway appears to play a role in only a small subset of sporadic medulloblastomas. Alternative molecular alterations would be important in the majority of sporadic medulloblastomas and may reflect the histologic heterogeneity of these tumors.