text: CMTM6 localizes with and binds to cell PD-L1 protein located on cell surface membranes thereby maintaining PD-L1'S expression at this site; it also localizes with PD-L1 protein located in recycling endosomes and thereby prevents PD-L1 from being degraded by lysosomal enzymes. These actions increase and maintain high levels of PD-L1 on cell surface membranes. PD-L1 protein on the surface of normal cells binds to PD-1 receptors on a type of cytotoxic T cells (i.e. CD8+ T cells) and thereby blocks these T-cells from organizing an immune response that would kill them. This PD-L1/CD8+ T cell circuit is one of several immune checkpoint mechanisms for maintaining self-tolerance, i.e. for preventing CD8+ T cells from attacking normal cells. Tumor cells may employ this immune-evading tactic: they may express PD-L1 and thereby block CD8+ T cell-mediated immune responses to themselves. In effect, the robust expression of PD-L1 helps not only normal cells but also cancer cells to evade immune destruction.