Monosomy 22 in rhabdoid or atypical tumors of the brain. Cytogenetic studies of three rare childhood brain tumors were performed. Two children presented with pure rhabdoid tumors. The third child had a tumor composed of a mixture of rhabdoid elements with neuroepithelial, epithelial, and mesenchymal tissue - an atypical teratoid tumor. All three tumors demonstrated monosomy 22 as the only cytogenetic abnormality. The cytogenetic findings suggest that loss of a gene or genes on chromosome 22 may be involved in the initiation or progression of these malignant tumors. Further studies on additional fresh tumor specimens are warranted; however, it is possible that cytogenetic studies may be used as an additional means of diagnosing rhabdoid or atypical teratoid tumors of the brain. 