Genetic predisposition to autoimmune diseases depends on many genetic loci, of which three play a key role: alleles of major histocompatibility complex genes (human leukocyte antigen- HLA), mainly class II HLA-DR3; alleles of a gene coding cytotoxic T lymphocyte associated antigen-4 (CTLA-4), and alleles of PTPN22 gene coding for lymphoid-specific tyrosine phosphatase (LYP) [1]. The gene discussed is CTLA4; the disease is autoimmune disease.