HTT and Huntington disease: N-terminal fragments containing exon 1 of huntingtin (Httex1) with expanded polyQ repeats, generated by either aberrant splicing or proteolytic cleavage, have been observed in human postmortem brains and mouse HD models (DiFiglia et al., 1997; Lunkes et al., 2002; Sathasivam et al., 2013; Neueder et al., 2017).