HLA-DRB1 and ependymoma: Among HLA class II genes, homozygosity frequencies for DRB1 and DQA1-DQB1 were low in nephroblastoma (none) and ependymoma (3.8%, a single case), respectively, but were in the higher range in osteosarcoma (13.2%, seven out of 53 cases), as compared to 8.8 and 10.2% of controls, respectively.