This paradox may be partly explained by the high level of PD-L1 on the placenta; similar to cancer cells, expressing PD-L1 can render cytotoxic T lymphocytes (CTLs) inactivated or nonfunctional through engagement of the inhibitory receptor PD-1, and the placenta expresses PD-L1 to escape T-cell attachment and induce immune tolerance to the foetus via PD-1 on T cells. This evidence concerns the gene CD274 and cancer.