The importance of Notch haploinsufficiency is underscored in studies of Alagille syndrome, which can be caused by heterozygous mutations in either JAG1 or NOTCH2 (Li et al., 1997; McCright et al., 2002; McDaniell et al., 2006; Turnpenny and Ellard, 2012; Huppert, 2016; Thakurdas et al., 2016). The gene discussed is JAG1; the disease is Alagille syndrome.