The protein TCP-1 (t complex polypeptide 1) is a subunit of the heterooligomeric complex CCT (chaperonin containing TCP-1) present in the eukaryotic cytosol. The CCT of eukaryotic cytosol is composed of eight different subunit species, TCP-1 (alpha), (beta), (gamma), (delta), E, ?, n and 0, each encoded by a different gene. Two (zeta)subunits have been described: TCP-1 (zeta)(also designated TCP-1 ?1) and TCP-1 ?2. TCP-1 subunits are proposed to have independent functions in folding its in vivo substrates, the actins and tubulins. TCP-1 was first identified in the mouse as relevant for tail-less and embryonic lethal phenotypes. Sequences homologous to TCP-1 have been isolated in several other species, and the yeast TCP-1 has been shown to encode a molecular chaperone for Actin and Tubulin. TCP-1 found in mammalian cells and yeast plays an important role in the folding of cytosolic proteins.