A subsequent study evaluating the role of PD-1 ligand for GVHD demonstrated that PD-1/PD-L1 blockade, but not PD-1/PD-L2 blockade, markedly accelerated GVHD lethality; suggesting an important differential role of host PD-L1 and PD-L2 in controlling GVHD (81). The gene discussed is PDCD1; the disease is graft versus host disease.