Chemokines are the largest family of cytokines in human immunophysiology. These proteins are defined by four invariant cysteines and are categorized based on the sequence around the first two cysteines, which leads to two major and two minor subfamilies.
Chemokine agonists that have an intervening amino acid between the first two cysteines are subclassified as CXC or α-chemokines. If the first two cysteines are adjacent to each other, the chemokines are known as CC or β-chemokines. Another subfamily, the CX3C or the γ -chemokine, possesses only one protein in its category and is defined by three intervening residues between the first two cysteines. The CX3C chemokine is unusual because it is part of a cell surface receptor. One of two exceptions to the four-cysteine paradigm is the C or δ-chemokine, in which the polypeptide has only two of the four cysteines. The same gene encodes both members of this subfamily, which share different splice sites and differ in only two amino acids. There are also a few chemokines from the β-chemokine subfamily that possess six cysteines, all of which are also disulfide bonded.
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