Actin is a globular, roughly 42-kDa multi-functional protein found in all eukaryotic cells (the only known exception being nematode sperm), where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved cytoskeletal proteins that play fundamental roles in nearly all aspects of eukaryotic cell biology. Actin polymerization is essential for cell locomotion and is thought to generate the force responsible for cellular protrusions.