As a critical post-translational modification, phosphorylation plays important roles in regulating various biological processes,Serine/threonine phosphorylation is an important mechanism that is involved in the regulation of protein function. Protein phosphorylation is the most well-studied post translational modification (PTM), in which a phosphoryl group from adenosine triphosphate (ATP) is covalently attached to a serine (~86%), threonine (~12%), or tyrosine (~2%) by a kinase and removed by a phosphatase. Phosphorylation at other amino acids have also been reported. Phosphorylation can modify protein structure, function, and interactions. As such, phosphorylation plays a critical role in virtually all cellular processes in homeostasis and disease, including signal transduction, cell cycle, differentiation, proliferation, metabolism, motility, and death. Importantly, phosphorylation at different residues can cause different outcomes. For example, RAF1 is a kinase central to the MAPK pathway that is activated when it is phosphorylated at serine (S) or threonine (T) residues S259, S338, S340/341, T491, or S494. However, phosphorylation at S289/296/301 results in the inhibition of RAF1 kinase activity.