This impairment occurs by the acquisition of mutations in cancer cells that, for example, lead to the expression of the PD-L1 and PD-L2 ligands (Kooi, Zhang, Patenia, Edwards, Platsoucas, Freedman, 1996, Hamanishi, Mandai, Iwasaki, Okazaki, Tanaka, Yamaguchi, Higuchi, Yagi, Takakura, Minato, Honjo, Fujii, 2007). This evidence concerns the gene PDCD1LG2 and cancer.