Ubiquitin is an ubiquitous 76 aminoacids protein that is present in all cellular compartments. It intervenes in numerous functions of cell metabolism, and in particular in non-lysosomal (but also lysosomal) lysis of altered or short-lived proteins. Immunohistochemical studies have shown that it is present in many inclusions characteristic of neurodegenerative diseases, notably in Lewy bodies, neurofibrillary tangles of Alzheimer's disease, Pick bodies and also in inclusions characteristic of certain motor neuron diseases. The presence of ubiquitin in these inclusions raises 2 questions: (1) the nature of the target proteins, probably altered proteins of the cytoskeleton; (2) the significance of ubiquination: it might reflect the degeneration process or participate in the protection of cells against degeneration or be an active factor in programmed cell death.