ST13 (suppression of tumorigenicity protein 13), also known as Hip (HSP70-interacting protein), is one of several co-chaperones that regulate activities of the HSP70 chaperone family. The homo-oligomeric protein Hip cooperates with HSP70 in protein folding by stabilizing the ADP-bound state of HSP70. Hip directly binds to the ATPase domain of HSP70 when it is converted to the ADP-bound state by proteins of the HSP40 family. By collaborating with other positive co-factors such as HSP40 and Hop, or competing with negative co-factors such as Bag1, Hip may facilitate the chaperone function of HSP70 in protein folding and repair, and in controlling the activity of regulatory proteins such as steroid receptors and various regulators of proliferation or apoptosis.