Although exchanges between sister chromatids are common in mitotic cells, those involving homologous chromosomes are rare. Since recombination between homologues is one of the functions of meiosis, it follows that one aspect of the differentiation of the meiocyte involves the synthesis of proteins or enzymes which facilitate synapsis and exchange. Mutants are known which seem to have constitutive levels of mitotic recombination between homologues, and these may be defective in the mechanism which normally represses mitotic recombination. It has been proposed that one component of the synaptonemal complex (s.c.) is a filamentous pairing protein with DNA binding sites which are base sequence specific. Synapsis occurs because the distribution of these sequences is the same in homologues. When only non-homologous chromsomes are present, as in haploid meiosis, only weak pairing can occur, since the base sequences are largely out of register. Although certain features of recombination at the molecular level are known, none of the models so far proposed suggest an explanation for interference between crossovers. It is suggested that interference may depend on the presence of a limited amount of another DNA binding protein which is specifically located within the s.c. A crossover between naked DNA molecules is initially a weak structure, which must be later converted into a visible and mechanically strong chiasma. It is assumed that this stabilization of a crossover is achieved by the DNA binding protein, which can diffuse freely within the s.c. and bind cooperatively to any recombinant DNA molecules within it. Depletion of the binding protein within the vicinity of a crossover makes it unlikely that the second crossover can be formed nearby.