During development, large numbers of cells die by a process known as programmed cell death. This loss of cells plays a number of important roles, including the sculpting of the body form and the removal of vestigial tissues. Data obtained from a variety of organisms has suggested that a cell's ‘decision’ to die is a differentiative event, requiring the activation of specific sets of genes. Several putative ‘cell death’ genes have recently been cloned, and one has been identified as the product of the polyubiquitin gene. Accumulation of ubiquitin has been observed not only during programmed cell death, but also in several neurodegenerative disorders, including Alzheimer's Disease.