Gelsolin is an actin-regulatory protein that modulates actin assembly and disassembly, and is believed to regulate cell motility in vivo through modulation of the actin network. In addition to its actin-regulatory function, gelsolin has also been proposed to affect cell growth.It is first isolated from rabbit lung macrophages as a modulator of the cytoplasmic actin gel−sol transformation. Gelsolin can sever actin filaments and cap the fast-growing ends of the filaments in vitro; this promotes actin disassembly. Gelsolin also has the ability to nucleate actin polymerization. These functions are activated by Ca² and inhibited by polyphosphoinositides (PPIs). Such observations have led to the view that gelsolin regulates actin reorganization in response to changes in the concentrations of Ca² and PPIs in living cells.