Heat shock protein 27 (Hsp27) also known as heat shock protein beta-1 (HSPB1) is a protein that in humans is encoded by the HSPB1 gene. Hsp27 is a chaperone of the sHsp (small heat shock protein) group among α-crystallin, Hsp20, and others. The common functions of sHsps are chaperone activity, thermotolerance, inhibition of apoptosis, regulation of cell development, and cell differentiation. They also take part in signal transduction. The main function of Hsp27 is to provide thermotolerance in vivo, cytoprotection, and support of cell survival under stress conditions. A well documented function of Hsp27 is the interaction with actin and intermediate filaments. Another function of Hsp27 is the activation of the proteasome. Probably Hsp27 – among other chaperones – is involved in the process of cell differentiation.