Signal transduction is a mechanism that converts a stimulus to a cell into a specific cellular response, through which the cell makes modifications in either activity of enzymes or gene transcription. Signal transduction processes are often initiated through receptor ligation or stresses, and may involve a cascade of signals within the cell. With each step of the cascade, the signal can be amplified and eventually creates a change to the cell to achieve the desired cell response. Transmembrane receptors span the cell membrane, with part of the receptor outside and part inside the cell. The chemical signal binds to the outer portion of the receptor, changing its shape and conveying another signal inside the cell. The cascading series of intracellular biochemical events often involve phosphorylation cascade. Activated kinases can catalyze the transfer of a phosphate group to serine, threonine, or tyrosine residues on target proteins and regulate protein activity, localization, and protein-protein interactions. Activities modified by phosphorylation can in turn be terminated by the phosphatases.