Unfortunately, no functional analyses have been performed to date on either eutopic or ectopic endometrial CD8 T cells, despite clear evidence of endometriosis-related dysfunctions in key regulators of CD8 T function, such as programmed cell death protein 1 (PD-1), programmed death-ligand 1 (PD-L1) and CD4+ T regulatory cells (29, 53, 95). This evidence concerns the gene CD4 and endometriosis.