Unrelated patients with MG were included in the study. They were enrolled in the Neurology Department of Beijing Tongren Hospital, Capital Medical University and fulfilled the clinical and electromyography diagnostic criteria for acquired MG. Simply, all MG patients met the following diagnostic criteria: typical symptoms of fluctuating muscle weakness, positive result of neostigmine test, and decremental response to low-frequency repetitive nerve stimulation. Information on age at onset, AChR / MuSK Abs status (partly), thymus status, involved muscles at onset and Osserman type at the maximum worsening during 2 years follow-up were obtained and used as the grouping basis of sub-classifications. They were genotyped for VDR rs1544410, rs2228570, rs731236, and rs7975232 polymorphisms

The geography and ethnically matched control group consisted of 146 unrelated healthy subjects. They were genotyped for VDR rs1544410, rs2228570, rs731236, and rs7975232 polymorphisms.

Genomic DNA was extracted from peripheral blood samples using a Wizard Genomic DNA Purification Kit (Promega, Madison,Wisconsin, USA) as per the product instruction. VDR (rs731236, rs1544410, rs7975232, rs2228570) polymorphisms were genotyped by the improved Multiple Ligase Detection Reaction (iMLDR) developed by Genesky Biotechnologies Inc. (Shanghai, China).

Codominant, dominant and recessive genetic models and haplotype analysis were applied to compare the difference between MG and control groups.

To study the precise effect of single nucleotide polymorphisms (SNPs) in the different population, the MG patients were divided into subgroups according to essential clinical variables, including onset age (≤50 or >50 years), thymus status (thymoma or non-thymoma, by pathology), AChR / MuSK Abs (positive or negative), onset involvement (ocular or generalized) and Osserman type at the maximum worsening (type I or IIa-IIb or III-V). The investigators analyzed the distribution of genes in each group.