Gm allotypes in myasthenia gravis.

Gm typing and acetylcholine receptor antibody assay were performed on serum samples from 74 patients with myasthenia gravis (31 male, 43 female) and from 236 unrelated normal blood-donors. The haplotype Gm1,2,21 was significantly more common in patients with myasthenia gravis (relative risk = 3.24), especially those with thymoma (relative risk = 6.99). The frequency of haplotype Gm1,2,21 was further increased in patients with severe generalised myasthenia gravis (relative risk = 10.52). The frequency of Gm1,2,21 was also increased in patients with high acetylcholine receptor antibody titres (greater than 5 pmol/ml). The results indicate the presence of a pathogenic gene close to the IgG heavy-chain gene complex in the 6th chromosome in the patients with myasthenia gravis, especially those with thymoma.