One patient developed persistent thrombocytopenia following completion of planned chemotherapy for AML and was referred for stem cell transplant; mild thrombocytopenia was noted during the screening work-up of her donor sister leading to the suspicion of an underlying familial platelet disorder; constitutional WES identified a germline mutation in RUNX1 leading to a change in donor selection and referral of the affected family members to the clinical genetic service. This evidence concerns the gene RUNX1 and Thrombocytopenia.