A regulatory site (also known as an "allosteric site") is a site on an allosteric protein to which a modulator molecule (or allosteric regulator) binds. This ligand-binding site on a receptor or enzyme is distinct from the active site. Allosteric modulators alter enzyme activity by binding to the regulatory site. Some allosteric regulators activate enzymes, while others inhibit them. The working hypothesis is that when allosteric regulators bind to the regulatory site, the protein undergoes conformational changes which changes the enzyme's activity.