Endoreduplication is a form of nuclear polyploidization that results in mul
tiple, uniform copies of chromosomes. This process is common in plants and
animals, especially in tissues with high metabolic activity, and it general
ly occurs in cells that are terminally differentiated, In plants, endoredup
lication is well documented in the endosperm and cotyledons of developing s
eeds, but it also occurs in many tissues throughout the plant. It is though
t that endoreduplication provides a mechanism to increase the level of gene
expression, but the function of this process has not been thoroughly inves
tigated. Numerous observations have been made of endoreduplication, or at l
east extra cycles of S-phase, as a consequence of mutations in genes contro
lling several aspects of cell cycle regulation. However, until recently the
re were few studies directed at the molecular mechanisms responsible for th
is specialized cell cycle. It is suggested that endoreduplication requires
nothing more elaborate than a loss of M-phase cyclin-dependent kinase activ
ity and oscillations in the activity of S-phase cyclin-dependent kinase.