PDCD1 and autoimmune disease: In this study, we extended earlier studies on the association of PD-1, PD-L1 or PD-L2 gene polymorphisms with autoimmune disease, and investigated whether the single-nucleotide polymorphisms (SNPs) rs2227981 and rs10204525 of PD-1, rs1970000 of PD-L1 and rs7854303 of PD-L2 could contribute to the development of Behcet's disease in a Chinese Han population.