HEY2 and isolated hemihyperplasia: Although the expression levels of the Notch components are likely dynamic during development, making transient expression difficult to detect, current data suggest that many known Notch components, mainly two ligands (Delta-like-4 and Jagged-1), three receptors (North-1, -3 and -4) and four effectors (HES-1, HEY1, HEY2 and HEYL) are involved in the pathogenesis of IH.