Limited capacity to replicate is a defining characteristic of most normal cells and culminates in senescence, an arrested state in which the cell remains viable but displays altered patterns of gene and protein expression. Senescent cells are not stimulated to divide by serum or passage in culture, and senescence invokes a specific cell cycle profile that differs from most damage-induced arrest processes or contact inhibition. An enlarged cell size, expression of pH-dependent b-galactosidase activity and an altered pattern of gene expression further characterize senescent cells.