Huntington disease (HD) belongs to a neurodegeneration disease family in which selective neuronal loss is caused by misfolded proteins. In HD, a polyglutamine (polyQ) expansion (>37 glutamines) in the N-terminal region of Huntington (htt) causes N-terminal htt fragments to misfold or aggregate, conferring neuropathology. Consistently, N-terminal htt fragments containing an expanded polyQ are able to interact with a number of proteins to mediate multiple pathological pathways. Thus, an important therapy approach is to inhibit the expression of mutant Huntington (htt) or its activity by blocking its abnormal interactions with other important proteins. Intracellular antibody (intrabody) provides a promising approach to achieve this goal, as intrabody is able to interact with an antigen intracellular to block its activity ortoxicity. However, normal htt is required for cell survival, and deletion of htt causes cell degeneration. Thus, a challenge for using the intrabody therapy for HD is to identify an intrabody that can selectively inhibit the toxicity of mutant htt but not interfere with the pivotal function of normal htt. Using the same antigen for an antibody (EM48) that reacts preferentially with misfolded and aggregated htt, we have established a hybridoma cell line that generates a monoclonal antibody (mEM48), which also preferentially reacts with mutant htt. By isolating the gene encoding mEM48 from this cell line, we have generated an intrabody that selectively binds mutant htt. We propose to characterize the protective effect of this intrabody on htt's toxicity in HD cellular models. We will also modify this intrabody to increase its expression level and stability. Finally, we will deliver this intrabody to HD mouse brain via viral vectors and to examine its protection against the neuropathology in HD brain. This proof of principle study will have a broad implication for the treatment of other neurodegeneration diseases that are also caused by misfolded proteins.