Within the context of the definitive adult blood system, alterations in RUNX1 dosage or activity have been associated with several blood-related disorders with both reduction (thrombocytopenia, myelodysplastic syndrome) and gain (Down syndrome hematopoietic disorders) of functional Runx1 alleles leading to abnormalities (Banno et al., 2016; De Vita et al., 2010; Rio-Machin et al., 2012; Song et al., 1999). The gene discussed is RUNX1; the disease is Down syndrome.