So let's discuss how they actually interact. So let's label this step as step number one. So, basically, the release of our calcium into the cytoplasm, into the cytosol of the cell, that is step number one. Now, what about step number two? Step number two is the binding of that calcium to a special protein found on our actin filament, on the thin filament known as troponin. So we have this protein that is shown by these three globular pink regions.