PDCD1 and cancer: For PD-1 polymorphisms, 54 case-control studies from a total of 26 articles [5,6,7,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,52] examining the associations of 6 widely studied polymorphisms in PD-1 gene and cancer risk were included in this meta-analysis.