The reversible phosphorylation of proteins represents a major post-translational signaling mechanism and regulatory pathway that controls a diverse set of cellular processes. The mode of action of these protein kinases is the reversible hydroxyl-phosphorylation of tyrosine, serine, and/or threonine residues of protein substrates. Since most kinases are specifically targeting tyrosine substrates, or serine/threonine substrates, the Axon Ligands™ in this catalogue targeting kinases have been subdivided into the two corresponding classes.Moreover, often the protein kinase itself is the substrate for an upstream kinase or undergoes auto-phosphorylation as part of a cascade of protein kinase signaling within the cell. Some representative protein kinase signaling pathways within cells include growth factor signaling and stress-activated signaling responses. Such pathways are highly interconnected and complex and regulate numerous cellular functions such as gene transcription, cell growth, proliferation, and differentiation. Signaling pathways of interest that will be highlighted individually are the DNA-damage response, the PI3K/AKT/mTOR pathway, the MAPK/ERK pathway, the NF-κB pathway, the TGF-β pathway, and the Wnt/β-Catenin pathway. Aberrant protein kinase activity can disrupt the normal control of cellular phosphorylation signaling pathways and lead to tumor formation. Given the critical role that protein kinases have in modulating cellular functions such as tumorigenesis, this class of enzymes has been targeted for the discovery and design of biologics and small-molecule inhibitors as potential therapeutic agents.