In eukaryotes, the phosphorylation and dephosphorylation of proteins on serine and threonine residues is an essential means of regulating a broad range of cellular functions, including division, homeostasis and apoptosis. A group of proteins that are intimately involved in this process are the protein phosphatases, specifically PP1 (protein phosphhatase 1), which is targeted to different substrates throughout the cell. Phostensin, also known as KIAA1949, is a 613 amino acid protein that localizes to both the cytoplasm and the cytoskeleton. Expressed predominately in spleen, ovary, lung and liver tissue, phostensin functions as a regulatory subunit that interacts with and targets PP1 to F-Actin in the cytoskeleton. Two isoforms of phostensin exist due to alternative splicing events.