Selective vulnerability of neurons is a critical feature of neurodegenerative diseases, but the underlying molecular mechanisms remain largely unknown. We here report that Omi/HtrA2, a mitochondrial protein regulating survival and apoptosis of cells, decreases selectively in striatal neurons that are most vulnerable to the Huntington's disease (HD) pathology. In microarray analysis, Omi/HtrA2 was decreased under the expression of mutant huntingtin (htt) in striatal neurons but not in cortical or cerebellar neurons. Mutant ataxin-1 (Atx-1) did not affect Omi/HtrA2 in any type of neuron. Western blot analysis of primary neurons expressing mutant htt also confirmed the selective reduction of the Omi/HtrA2 protein. Immunohistochemistry with a mutant htt-transgenic mouse line and human HD brains confirmed reduction of Omi/HtrA2 in striatal neurons. Overexpression of Omi/HtrA2 by adenovirus vector reverted mutant htt-induced cell death in primary neurons. These results collectively suggest that the homeostatic but not proapoptotic function of Omi/HtrA2 is linked to selective vulnerability of striatal neurons in HD pathology.