There is now a large body of evidence in support of the view that Ca2+ binding to the low affinity sites of TnC induces a movement of helices B and C away from helices A and D, thus opening a hydrophobic cavity, the site of interaction with TnI. Another site of similar structure is formed by the helical segments in the C-terminal domain. Both sites appear to interact with the inhibitory segment of TnI. Whereas the interactions at both sites are necessary for the full regulatory activity of TnC, the interaction at the C-terminal domain stabilizes the complex and that involving the N-terminal domain is directly linked to the release of inhibition. In the absence of Ca2+ the inhibitory region of TnI would preferentially bind to actin and on Ca2+ binding to sites I and II it would switch to the site in the N-terminal domain of TnC. Detachment of TnI from actin gives rise to further events in thin filament regulation.