Induction of asymmetry into homodimers. 
The self-regulation of biological signalling receptors via homodimerization is discussed in relation to the symmetry changes occurring when these receptors bind their target ligand.  The idea of positive and negative cooperativity between dimerization and ligand binding, mediated by changes in the symmetry of the system as a source of signalling control is considered; an analogy made with the homodimerization of a glycopeptide antibiotic, ristocetin A, which displays negative cooperativity.  Finally, the regulation of the bacterial aspartate receptor and the human growth hormone receptor is discussed as a function of ligand-induced asymmetry.