The mechanism of the early onset of AD pathology in DS is not fully understood, but the over-expression of certain genes on Hsa21, particularly in the Down syndrome critical region 1 (DSCR1) that also includes the amyloid precursor protein (APP), Cu/Zn superoxide dismutase (SOD1), RUNX1 (a transcription factor) [7] and S100β, play crucial roles [8,9]. The gene discussed is RUNX1; the disease is Alzheimer disease.