HLA-DRB1 and thymoma: In order to assess this heterogeneity, a case-control association study in the Norwegian population examined the HLA-A, -B, -C, and  -DRB1 loci in well-defined subgroups of MG patients sorted by the age at onset (Early-onset < 40 years, Late-onset > 60 years and intermediate group = 41–59 years) and the histology of the thymus (thymoma or nonthymoma).