Differently from conventional primary neuroectodermal tumors (PNETs), molecular features of undifferentiated lesions have been poorly studied. Medulloblastoma and PNET neoplasms showed a high incidence of loss of heterozygosity (LOH) on chromosome 17p13, in the region of tumor suppressor gene p53. Recent studies have shown a significant correlation between the presence of p53 Arg72Pro polymorphism and several undifferentiated carcinomas. We performed molecular analysis in an anaplastic tumor of posterior fossa in a patient with a constitutional maternal translocation [46,XX,t(5;19)] and a history of headache, nausea and vomiting. We identified the presence of LOH at 17p13 and Pro72Arg polymorphism in tumor DNA. These molecular findings helped us better characterize this undifferentiated tumor and led to a more aggressive therapy.