HLA class II antigens and DNA restriction fragment length polymorphism in myasthenia gravis in Japan. Human leukocyte phenotypes and genes in the HLA class II regions were studied in 46 Japanese patients with myasthenia gravis. When the HLA phenotypes of the patients with myasthenia gravis were compared with the controls, an increased frequency of HLA-DRw53 was observed in females less than 30 years of age. The genomic DNAs of the HLA-DRw53-positive patients and DRw53-positive controls were analyzed by using four complementary DNA probes for HLA class II genes. With DQB complementary DNA as the probe, a higher incidence of the 6.5-kb or 8.2-kb BamHI fragment was observed in the patients (76.0%) compared with the controls (19.0%). In contrast, no significant difference was observed between patients and controls when complementary DNAs for DRB, DQA, and DPB were used as probes. These results indicate that the genetic background of Japanese females with early-onset myasthenia gravis is different from other patients with myasthenia gravis, and that DQB genes can greatly influence the onset of myasthenia gravis. 