HTT and juvenile Huntington disease: Aggregates are generated when proteins become destabilized, either by mutations changing their native state (e.g. SOD1 in ALS) or quantity (e.g. α-synuclein in PD), by the elongation of a certain domain [e.g. huntingtin (Htt) in Huntington’s disease (HD)] or by domain truncations (e.g. TDP-43 in ALS).