Phosphorylation is the addition of a phosphoryl group (PO3)? to a molecule. In biology, phosphorylation and its counterpart, dephosphorylation, are critical for many cellular processes. A large fraction of proteins (between 1/3 - 2/3 of the proteome in eukaryotes) are temporarily phosphorylated, as are many sugars, lipids, and other molecules. Phosphorylation is especially important for protein function as this modification activates (or deactivates) almost half of the enzymes, thereby regulating their function.