HTT and Huntington disease: Moreover, it has been suggested that not all Htt proteolytic fragments contribute to toxicity and therefore pathogenic as well as a nonpathogenic role has been suggested for the Htt cleavage by different caspases; for example, using mass spectrometry and site-directed mutagenesis, it has been recently demonstrated that the generation of a cp-2 fragment (specifically cleaved at the level of arginine 167) produces a pathogenic fragment; this study reveals that specific sites of Htt are involved in the toxicity produced by the mutant Htt in HD [20].