Gelsolin is a calcium dependent actin binding protein. It is a potent modulator of actin filament length and gelation. Gelsolin has been shown to exist in at least two variant forms, cytoplasmic gelsolin and plasma gelsolin. Plasma gelsolin has also been called actin depolymerizing factor (ADF) or brevin. Human plasma and rabbit macrophage gelsolins differ by the presence of a 25 amino acid residue extension at the NH2 terminus in the human plasma gelsolin, which appears to account for the difference in relative molecular weights.