The majority of chemokines are highly basic, small proteins, with a molecular mass of around 8–10 kDa. Although they do not necessarily have a high level of homology at the primary sequence level, which can be as low as 20%, although it can also be as high as 90%, the three-dimensional structure of all the chemokines solved to date has shown that they have a superimposable monomeric fold. This fold is imposed on the family by the four cysteine motif that is common to the majority of the chemokines since they all form two disulfides between Cys-1 and 3, and Cys2 and 4, whether they belong to the α or CXC or β or CC subclass. Molecular modeling of the two chemokines that deviate from the motif, the C chemokine lymphotactin which lacks a disulfide, and the CX₃C chemokine neurotactin or fraktalkine which has three amino acids between the first cysteine pair, both adopt the same fold. Modeling in fact shows that either one or three residues between the first two Cys can be adopted, but not two, probably explaining why examples of CX₂C chemokines do not exist.