HTT and Huntington disease: Indeed, Htt knockout in mice results in early embryonic lethality (Duyao et al, 1995; Nasir et al, 1995; Zeitlin et al, 1995) and hypomorphic alleles that express less Htt than normal in humans cause a profound neurodevelopmental disorder whose symptoms are distinct from the HD (Rodan et al, 2014; Lopes et al, 2016).