Protein aggregation seems to be a common feature of several neurodegenerative diseases and to some extent of physiological aging. It is not always clear why protein aggregation takes place, but a disturbance in the homeostasis between protein synthesis and protein degradation seems to be important. The result is the accumulation of modified proteins, which tend to form high molecular weight aggregates. Such aggregates are also called inclusion bodies, plaques, lipofuscin, ceroid, or 'aggresomes' depending on their location and composition.

One of the highlights of postmitotic aging is the intracellular accumulation of highly oxidized and cross-linked proteins, known as lipofuscin. Lipofuscin is insoluble and not degradable by lysosomal enzymes or the proteasomal system, which is responsible for the recognition and degradation of misfolded and oxidatively damaged proteins. These aggregates have been found in various cell types, including heart, liver, kidney, neuronal tissue, and dermal tissue, and are associated with the life span of a single postmitotic cell and, consequently, of the whole organism.

Lipofuscin formation appears to depend on the rate of oxidative damage to proteins, the functionality of mitochondrial repair systems, the proteasomal system, and the functionality and effectiveness of the lysosomes. Such aggregates are not inert metabolic end products, but actively influence the metabolism of cells, in particular proteasomal activity and protein turnover.