Hsp20 is a small heat shock protein related to Hsp25, Hsp27 and may form different hetercomplexes with these proteins. The specific physiological function of Hsp20 is not yet known. It is distributed ubiquitously in tissues, but is found in higher levels in skeletal, smooth and heart muscle. Under normal conditions, Hsp20 is diffusely distributed in the cytosol, but under heat stress conditions, it translocates to the nucleus. Unlike other heat shock proteins the amount of Hsp20 does not increase after heat shock. The Hsp20 was demonstrated to constitute up to 1.3% of the total cellular protein in vertebrate tissues, especially in muscle, and its expression is related to muscle contraction, specifically in slow-twitch muscle. Hsp20 may form different heterocomplexes with other Hsp's, such as alpha-crystalline and Hsp25. Phosphorylated form of Hsp20 is proposed to interact with monomeric actin whereas dephosphorylated form binds polymeric actin filaments. In normal conditions Hsp20 is diffusely disturbed in cytosol but under the heat stress it undergoes translocation to membrane fraction.