The adult Caenorhabditis elegans nematode, a small roundworm, has a precisely defined number of somatic cells that create organs that are also found in larger animals, including intestine, muscles, skin, an excretory system and a primitive brain. Every cell has a defined role in this sophisticated, but tiny animal. Therefore, stringent control of the cell cycle is required to produce the almost invariant cell lineage that generates the C. elegans somatic body plan. The proliferation of germ cells is regulated differently, and occurs within a stem cell niche.