Olaparib inhibits PARP-2 (50% inhibitory concentration of PARP-2:1–251 nM).[23] In vivo studies have revealed that PARP-2 plays an essential role in the survival of hematopoietic stem/progenitor cells,[24,25] indicating that loss of PARP-2 function leads to macrocytic anemia, impairing DNA metabolism, even with increased erythropoietin levels.[25] Therefore, PARP-2 may be involved in the main mechanism of olaparib-induced anemia. The gene discussed is PARP2; the disease is macrocytic anemia.