Molecular studies from the past decade have revealed striking conservation in the mechanisms of eukaryotic cell cycle control. Yet before the advent of molecular genetics, it was clear that eukaryotes possessed many different cell cycle variations, and thus that there must be diversity in mechanisms of control. One common cell cycle variant is the endoreplication cycle, in which cells increase their genomic DNA content without dividing. Although endocycles are sometimes dismissed as an evolutionary peculiarity, they are widespread in protists, plants, and many animals including arthropods, mollusks, and mammals. Endocycling cells can become incredibly polyploid, with chromatin values (C values denote DNA content as a multiple of the normal haploid genome) as high as 24,000 reported in some plant endosperms (