Many of the Axon Ligands™ in this class of compounds target receptors of various growth factors, such as EGF, VEGF, and PDGF. These receptors are members of the class of enzyme linked receptors, which, as integral membrane proteins, possess both receptor functionality (extra-cellular) as well as enzymatic catalytic functionality (intracellular). The majority of the enzymatic activity of this class of receptors is characterized by kinase-like activity. Based on this feature, five main classes can be distinguished: Receptor Tyrosine Kinases (RTKs), and Receptor Serine/Threonine Kinases (RSTKs, participating in MAPK and TGF-beta signaling pathways, among others) are well known. Additionally, there are classes of Receptor Guanylyl Cyclases, Histidine Kinase associated Receptors (receptors that associate with proteins that have histidine kinase activity), and finally a class of Tyrosine Kinase associated Receptors (e.g. Cytokine Receptors). In addition, some transmembrane tyrosine phosphatases (Receptor-like) Protein Tyrosine Phosphatases (PTPs)), which remove phosphate from phosphotyrosine side chains of specific proteins, are thought to function as receptors, although for the most part their ligands are unknown. Within each of these main classes, sub-classes exist, based on the specific endogenous ligands. Many of the enzyme linked receptors play a role in the regulation of cell proliferation, programmed cell death (apoptosis), cell differentiation, and embryonic development, and therefore are of great interest as targets for the treatment of cancer. Furthermore, malfunctioning of receptors of this kind is associated with the development of neurodegenerative diseases, such as multiple sclerosis and Alzheimer's disease.