High-dose cyclophosphamide given early after transplant effectively prevents alloreactivity (GvHD and graft rejection) and to spare stem cells, allowing successful mismatched donor transplant. Hematopoietic and other tissue stem cells, including memory T cells, express high levels of aldehyde dehydrogenase 1 (ALDH1), the body's primary means of inactivating cyclophosphamide, whereas mature lymphocytes generally express low levels. Figure reproduced by Andrea Sikora, PharmD with permission; modification from Expert Rev Hematol. 2019;12(9):733-752.