Microcins are very small bacteriocins, composed of a relatively few peptides. For this reason, they are distinct from their larger protein cousins. The classic example is microcin V, of E. coli. Subtilosin A is another bacteriocin from Bacillus subtilis. The peptide has a cyclized backbone and forms three cross-liks between the sulphurs of Cys13, Cys7 and Cys4 and the alpha-positions of Phe22,Thr28 and Phe31.