Medulloblastomas (MBs) are malignant primitive neuroectodermal tumours (PNETs) of the cerebellum occurring predominantly in childhood. The association of monosomy of chromosome 22 with MB is controversial. Atypical teratoid/rhabdoid tumours (AT/RTs) of the brain share clinical and histological features with MBs and supratentorial PNETs (sPNETs). In particular, AT/RTs can be misdiagnosed as MBs and sPNETs because AT/RTs frequently contain areas of primitive neuroepithelial cells similar to PNETs. Recently, mutations of the tumour suppressor gene hSNF5/INI1, located on 22q11.23, have been described in AT/RTs, MBs and sPNETs, with conflicting data on the prevalence of hSNF5/INI1 mutations in the latter entities. Therefore, we screened MBs for point mutations and homozygous deletions of the hSNF5/INI1 tumour suppressor gene. In 90 MBs, no mutations of the hSNF5/INI1 gene were identified. Thus, our study virtually rules out hSNF5/INI1 as a tumour suppressor gene involved in the pathogenesis of medulloblastoma.