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In the past twenty years, evidence has accumulated to show that ubiquitinated proteins are a consistent feature of the intraneuronal protein aggregates (inclusions) that characterize chronic neurodegenerative disease. These findings may indicate that age-related dysfunction of the 26S proteasome may be central to disease pathogenesis. The aggregate-prone proteins can also be eliminated by autophagy. We have used the Cre-recombinase/loxP genetic approach to ablate the proteasomal Psmc1 ATPase gene and deplete 26S proteasomes in neurons in different regions of the brain to mimic neurodegeneration. Deletion of the gene in dopaminergic neurons in the substantia nigra generates a new model of Parkinson disease. Ablation of the gene in the forebrain creates the first model of dementia with Lewy bodies. In both neuroanatomical regions, gene ablation causes the formation of Lewy-like inclusions together with extensive neurodegeneration. There is some evidence for neuronal autophagy in areas adjacent to inclusions. The models indicate that neuronal loss in neurodegenerative diseases can be attributed to proteasomal malfunction accompanied by Lewy-like inclusions as seen in dementia with Lewy bodies and Parkinson disease.