For several decades, allogeneic hematopoietic cell transplantation (HCT) has remained an important tool in the management of patients with high-risk hematological malignancies. Reduced intensity conditioning regimens and advances in supportive care have improved the safety and accessibility of HCT and also increased the need for a deep and diverse stem cell donor pool. This demand has highlighted existing disparities in the unrelated donor registry, particularly for ethnic minorities, who face significant challenges in identifying HLA-matched adult donors. 1 The acceptance of umbilical cord blood and haploidentical grafts as viable 'alternative' donors has increased the options for these patients and now ensures that a donor can be identified for virtually all patients.The utilization of haploidentical grafts represents appealing options owing to their ubiquity, low cost and donor availability for future lymphocyte accession compared with umbilical cord blood units. Haploidentical grafts, however, lack the long-term follow-up of traditional stem cell sources resulting in several areas where the optimal management strategy remains unknown. With a growing number of patients receiving HCTs from haploidentical sources in the United States and abroad, advancing the care of these patients represents an area of tremendous interest and clinical necessity. 2 Two decades ago, Beatty et al. 3 first reported their experience with HLA-disparate allografts. The patients had high rates of graft rejection and acute GVHD (aGVHD) that was nearly double the HLAmatched control group. Over the following decade, animal studies with post-HCT cyclophosphamide demonstrated encouraging results and gave way to two early-phase studies. [4][5][6] Mechanistically, post-transplant cyclophosphamide reduces the robust, alloantigendirected, naive T-lymphocyte population after stem cell infusion. 7 With posttransplant cyclophosphamide, the rates of aGVHD are substantially reduced. 8,9 Subsequent work in 2008 confirmed minimal aGVHD; however, more than half of patients relapsed within 1 year and 2-year overall survival was a staggering 36%. 10 Parallel, multicenter trials were published 3 years later comparing double umbilical cord blood units with haploidentical grafts. After a median follow-up of 357 days, 45% of haploidentical patients relapsed and 62% were still alive 1 year after HCT. 11 The use of post-transplant cyclophosphamide was championed as a measure that advanced the safety of haploidentical HCT through diminished rates of aGHVD. GVHD, and the coveted GVL effects, are dogmatically linked and drive the long-term remissions seen in survivors. 12,13 In spite of these low rates of aGVHD, improvements in the post-HCT management of these patients translated into improved clinical outcomes. 10,[14][15][16][17][18][19][20][21][22][23][24] Relapsed disease is a principal threat to these patients' longterm survival and affects 30-50% of our patients. [25][26][27] Relapse is driven by an aggressive disease biology that can be difficult and in m...