Bone marrow failure unresponsive to bone marrow transplant is caused by mutations in .

We report 5 individuals in 3 unrelated families with severe thrombocytopenia progressing to trilineage bone marrow failure (BMF). Four of the children received hematopoietic stem cell transplants and all showed poor graft function with persistent severe cytopenias even after repeated transplants with different donors. Exome and targeted sequencing identified mutations in the gene encoding thrombopoietin (): THPO R99W, homozygous in affected children in 2 families, and THPO R157X, homozygous in the affected child in the third family.