Actin is a globular multi-functional protein that forms microfilaments. It is found in essentially all eukaryotic cells (the only known exception being nematode sperm), where it may be present at concentrations of over 100 µM. An actin protein's mass is roughly 42-kDa and it is the monomeric subunit of two types of filaments in cells: microfilaments, one of the three major components of the cytoskeleton, and thin filaments, part of the contractile apparatus in muscle cells. It can be present as either a free monomer called G-actin (globular) or as part of a linear polymer microfilament called F-actin (filamentous), both of which are essential for such important cellular functions as the mobility and contraction of cells during cell division.