Background: Germline RUNX1mutation is associated with an autosomal dominant familial platelet disorder with associated myeloid malignancy (FPDMM), most commonly myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML), but lymphoblastic leukemias are also reported. FPDMM is characterized by thrombocytopenia from an early age associated with abnormal platelet function. Herein we report the bone marrow features, in addition to laboratory and clinical findings, of a large pediatric cohort (N=29) with RUNX1-FPDMM. These data have value in guiding early investigation of neonatal/pediatric thrombocytopenia, and baseline bone marrow interpretation to avoid overdiagnosis of MDS or misdiagnosis of immune-mediated thrombocytopenias.