Huntington's Disease belongs to the CAG repeat family of neurodegenerative diseases and is characterized by the presence of an expanded polyglutamine (polyQ) repeat in the huntingtin (htt) gene product. PolyQ-expanded htt accumulates within large aggregates that are found in various subcellular compartments, but are more often localized within the nucleus. It has been suggested that the sequestration of proteins essential to cell viability may be one mechanism that accounts for toxicity generated by polyQ-expanded proteins. Nuclear inclusions containing polyQ-expanded htt recruit the transcriptional cofactor, CREB-binding protein (CBP). PolyQ toxicity appears to involve alterations of gene transcription and reduced neuronal cell viability. In the HT22 hippocampal cell line, we find that toxicity within individual cells induced by polyQ-expanded htt, as revealed by a TUNEL assay, is associated with the localization of the mutant htt within either nuclear or perinuclear aggregates. However, in addition to CBP recruitment, we show here that CBP ubiquitylation and degradation can be selectively enhanced by polyQ-expanded htt. Thus, selected substrates may be directed to the ubiquitin/proteasome-dependent protein degradation pathway in response to polyQ-expanded htt within the nucleus.