PPM1D and cyclic hematopoiesis: Given that PPM1D mutations have been associated with CH in patients with prior exposure to cytotoxic therapy (Coombs et al., 2017, Wong et al., 2018), we began our investigation with the therapy-related acute myeloid leukemia (t-AML) and therapy-related myelodysplastic syndrome (t-MDS) that arise in some individuals years after chemotherapy for solid tumors or non-myeloid hematologic malignancies.