HIF1A and isolated hemihyperplasia: The three leading hypotheses on the pathogenesis of IH are (1) local hypoxemia leading to hypoxia-inducible factor 1 alpha (HIF-1α)-induced proliferation12,13, (2) embolization of placental cells14–17, and (3) vasculogenesis/angiogenesis driven by hypoxemia-induced differentiation of mesenchymal stem cells into endothelial cells and Notch-mediated differentiation of mesenchymal stem cells into proangiogenic pericytes18–22.