text: Proteins are classified into the chemokine family based on their structural characteristics, not just their ability to attract cells. All chemokines are small, with a molecular mass of between 8 and 10 kDa. They are approximately 20-50% identical to each other; that is, they share gene sequence and amino acid sequence homology. They all also possess conserved amino acids that are important for creating their 3-dimensional or tertiary structure, such as (in most cases) four cysteines that interact with each other in pairs to create a Greek key shape that is a characteristic of chemokines. Intramolecular disulfide bonds typically join the first to third, and the second to fourth cysteine residues, numbered as they appear in the protein sequence of the chemokine.  Typical chemokine proteins are produced as pro-peptides, beginning with a signal peptide of approximately 20 amino acids that gets cleaved from the active (mature) portion of the molecule during the process of its secretion from the cell.  The first two cysteines, in a chemokine, are situated close together near the N-terminal end of the mature protein, with the third cysteine residing in the centre of the molecule and the fourth close to the C-terminal end.  A loop of approximately ten amino acids follows the first two cysteines and is known as the N-loop.   This is followed by a single-turn helix, called a 3-helix, three β-strands and a C-terminal α-helix.  These helices and strands are connected by turns called 30s, 40s and 50s loops; the third and fourth cysteines are located in the 30s and 50s loops.