This is an open label phase II study in patients undergoing allogeneic hematopoietic stem cell transplantation and receiving standard sirolimus and tacrolimus GvHD prophylaxis. Although the myeloablative preparative regimen is not prescribed, it is anticipated that most patients will receive total body irradiation (TBI) plus etoposide (TBI/VP16), or high-dose thiotepa plus cyclophosphamide according to institutional standards. Regardless of the preparative regimen, all patients will receive the following regimen for GvHD prophylaxis, which includes the study drug sitagliptin: