Huntington's disease (HD) mouse models that express N-terminal huntingtin fragments show rapid disease progression and have been used for developing therapeutics. However, light microscopy reveals no significant neurodegeneration in these mice. It remains unclear how mutant huntingtin induces neurodegeneration. Using caspase staining, terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling, and electron microscopy, we observed that N171-82Q mice, which express the first 171 aa of mutant huntingtin, displayed more degenerated neurons than did other HD mouse models. The neurodegeneration was also evidenced by increased immunostaining for glial fibrillary acidic protein and ultrastructural features of apoptosis. R6/2 mice, which express exon 1 of mutant huntingtin, showed dark, nonapoptotic neurons and degenerated mitochondria associated with mutant huntingtin. In HD repeat knock-in mice (HdhCAG150), which express full-length mutant huntingtin, degenerated cytoplasmic organelles were found in both axons and neuronal cell bodies in association with mutant huntingtin that was not labeled by an antibody to huntingtin amino acids 342-456. Transfection of cultured cells with mutant huntingtin revealed that an N-terminal huntingtin fragment (amino acids 1-208 plus a 120 glutamine repeat) caused a greater increase in caspase activity than did exon 1 huntingtin and longer huntingtin fragments. These results suggest that context-dependent neurodegeneration in HD may be mediated by different N-terminal huntingtin fragments. In addition, this study has identified neurodegenerative markers for the evaluation of therapeutic treatments in HD mouse models.