Haploinsufficiency of the human 17p13.3 region is associated with 35% to 50% of medulloblastomas, indicating the presence of one or more tumour suppressor genes which have not yet been identified. Of 119 genes residing in this region, seven genes--14-3-3epsilon (YWHAE), HIC-1, ROX/MNT (a helix-loop-helix transcription factor and member of the MYC/MAX superfamily), KIAA0399, UBE2G1 (ubiquitin ligase), ALOX15, and MINK--encode proteins with potential links to cancer. We investigated these genes and found significant levels of expression of ROX/MNT in adult human cerebellum, and in embryonic and postnatal mouse cerebellum. Six of 14 medulloblastomas showed a reduction of ROX/MNT expression, accompanied by a reduction of both UBE2G1 and 14-3-3epsilon in three tumours and a reduction of UBE2G1 in one tumour. Moreover, the relative expression of MYC to ROX/MNT was increased in 4 of the 14 medulloblastomas. Collectively, these data suggest that ROX/MNT should be considered a potential tumour suppressor gene in medulloblastoma.