Chemokine families. Chemokines are divided into families based on structural and genetic considerations. All chemokines are structurally similar, having at least three β-pleated sheets (indicated as β1-3) and a C-terminal α-helix. Most chemokines also have at least four cysteines in conserved positions. In the CXC chemokine family, the two cysteines nearest the N-termini of family members are separated by a single (and variable) amino acid. The genes encoding these proteins cluster at human chromosome 14q12-21 (except for SDF-1α whose gene maps to chromosome 10⁵³). In the CC chemokine family, the two cysteines nearest the N-termini of these proteins are adjacent. Their genes cluster at 17q11.2-12 (except for MIP-3β, whose gene maps to chromosome 9¹¹⁷ and MIP-3α/LARC which maps to chromosome 2^117a). Lymphotactin is a structurally related chemokine having only one cysteine near its N-terminus and is said to belong to the C chemokine family. The CX₃C chemokine (also called “fractalkine” or “neurotactin”) has a typical chemokine-like structure at its N-terminus except for the placement of three amino acids between the first two cysteines. This chemokine domain occurs at the end of a long stalk which is heavily substituted with mucin-like carbohydrates. The protein is embedded in the membrane and has a short cytoplasmic domain.