To further assess the cell-intrinsic inhibitory phenotype of PD-1–overexpressing T cells in vivo, we used a graft-versus-host disease (GVHD) model that allows us to track and measure polyclonal, alloreactive T cell proliferation and activation in vivo after adoptive transfer into an allogeneic recipient (17). The gene discussed is PDCD1; the disease is graft versus host disease.