The precise mechanisms that control the aging process and determine life span remain unknown. In general, however, theories of aging fall into two major categories. One group is described as stochastic because these theories are dependent upon time and probability. The "error-catastrophe" or "genetic wear and tear" theories of aging, for example, postulate that random errors of protein synthesis due to faulty nucleic acid transcription or translation or a simple degradation of the integrity of nuclear DNA (nDNA) eventually compromise cellular and tissue function and produce the physical signs of aging. In contrast, the non-stochastic group of theories proposes that programmed or predetermined mechanisms are responsible for aging. In order to produce change throughout the organism, a pacemaker tissue or organ must have widespread interaction with all other organ systems. Therefore, this type of theory usually requires that neuroendocrine or immune mechanisms play a central role in processes of aging.