Several reports demonstrated in mice (12, 29) and humans (7, 30) that CD8 T cells infiltrating the bone marrow after allogeneic HSCT express significantly higher levels of PD-1 at their surface compared to peripheral blood cells, a difference that might derive from the interaction with the micro-environment and/or with tumor cells in case of disease persistence or relapse. The gene discussed is PDCD1; the disease is neoplasm.