In cellular biology, proteins act as intracellular signaling molecules by activating another protein in a signaling pathway. In order to do this, proteins can switch between active and inactive states, thus acting as molecular switches in response to another signal. For example, phosphorylation of proteins can be used to activate or inactivate proteins. The external signal flipping the molecular switch could be a protein kinase, which adds a phosphate group to the protein, or a protein phosphatase, which removes phosphate groups.