Huntington disease (HD) is a neurodegenerative disorder caused by a genetic defect in Huntingtin gene (<i>HTT</i>) that leads to the expression of an expanded poly-glutamine protein. HD has a broad impact on a person's functional abilities due to progressive motor dysfunction, cognitive decline, and psychiatric disturbance, probably caused by both neuronal dysfunction and neuronal cell death. New strategies for drug discovery and new therapeutic approaches are now leading to slow the progression of HD; one of these promising strategies consists in reducing the mutant protein level through its direct reduction (gene silencing) or increasing its clearance (compound treatment). In this framework mutant HTT protein can be a biomarker for treatment efficacy and disease progression, raising the necessity to develop highly sensitive immunoassays to quantify its level in biological fluids. The Single Molecule Counting (SMC?) technology, powered by Singulex?, enabled the ultrasensitive measurement of mutant HTT at levels previously undetectable in human Cerebrospinal fluid (CSF). The immunoassay was developed using the 2B7 and the MW1 antibodies that recognize the N-terminal region and the poly-Q trait of the protein respectively and was successfully applied to human and murine derived fluids and murine tissues. Here is presented a summary of results obtained from the assay validation process, including the evaluation of calibration curve performance, specificity, matrix effect (spike recovery, parallelism and dilution linearity), selectivity, accuracy, precision and stabilit
