The catecholamines dopamine (DA), norepinephrine (NE) and epinephrine are neurotransmitters and/or hormones in the periphery and in the central nervous system (CNS). NE is a neurotransmitter in the brain as well as in postganglionic, sympathetic neurons. DA, the precursor of NE, has biological activity in the periphery, most particularly in the kidney, and serves as a neurotransmitter in several important pathways in the CNS. Epinephrine, formed by the N-methylation of NE, is a hormone released from the adrenal gland, and it stimulates catecholamine receptors in a variety of organs. Small amounts of epinephrine are also found in the CNS, particularly in the brainstem.

Cathelicidins act as a primary defense against bacteria and other pathogens in the case of inflammation. They are able to kill bacteria and fungi, inhibit and destroy bacterial biofilms, and possess antiviral and antiparasitics properties. They can also play a role in angiogenesis, wound healing, and the regulation of apoptosis. Cathelicidins are able to directly and selectively destroy membranes of various microbes and cancer cells, but they do not attack normal cells.

Cathepsin K is a unique and potent collagenase primarily expressed in osteoclasts. Inhibition of this enzyme abrogates bone resorption, highlighting the potential clinical benefit of targeting cathepsin K activity.

Chemokines are a family of small and secreted proteins that play pleiotropic roles in inflammation-related pathological diseases including cancer. Among the identified 50 human chemokines, chemokine (C-C motif) ligand 2 (CCL2) is of particular importance in cancer development since it serves as one of the key mediators of interactions between tumor and host cells.

Chemokine (C-C motif) ligand 15 (CCL15) is a small cytokine belonging to the CC chemokine family that is also known as leukotactin-1, MIP5 and HCC-2. CCL15 is expressed i liver, small intestine, colon, and in certain leukocytes and macrophages of the lung. It is chemotactic for neutrophils, monocytes, and lymphocytes and elicits its effects by binding to cell surface chemokine receptors like CCR1 and CCR3.

Chemokine (C-C motif) ligand 18 (CCL18) is a small cytokine belonging to the CC chemokine family. The receptor for CCL18 has been identified in humans only recently, which will help scientists understand the molecule's role in the body.

CCL18 is produced and secreted mainly by innate immune system, and has effects mainly on the adaptive immune system.

Non-enzymatic chitinase-3 like-protein-1 (CHI3L1) belongs to glycoside hydrolase family 18. It binds to chitin, heparin, and hyaluronic acid, and is regulated by extracellular matrix changes, cytokines, growth factors, drugs, and stress. CHI3L1 is synthesized and secreted by a multitude of cells including macrophages, neutrophils, synoviocytes, chondrocytes, fibroblast-like cells, smooth muscle cells, and tumor cells. It plays a major role in tissue injury, inflammation, tissue repair, and remodeling responses.

The circulatory level of CAF may represent an early indicator of NMJ dismantling and muscle fiber denervation, which signals the onset of sarcopenia.

CTX is a degradation product of the type I collagen. The latter is a triple helix made up of two α1-chains and one α2-chain. The chains have a helical section thanks to the repetition, in their primary structure, of three amino acids: Gly-X-Y in which Y is often a proline or a hydroxyproline increasing the molecule stability while X can be either a proline or lysine. Triple helix structures are stabilized by covalent links. These covalent links are called cross-links. CTX is thus a degradatoion product of type 1 collagen, containing a cross-lap.

Cartilage tissue mainly consists of type II collagen which forms the fibrillar structure of the extracellular matrix. Type II collagen is constituted by the association of three identical α1 chains in a triple helix except at the two ends, in the N- and C-telopeptides. Proteolytic cleavage of type II collagen involves several proteases including matrix-metalloproteases (MMPs) and cathepsins (Cats) which are synthesized by the chondrocytes and synovial cells in response to cytokines and growth factors. CTXII is a product of degradation coming from Type II collagen. It is composed of a crosslap.