Potential roles of BRCA2 in promoting assembly of Rad51 at sites of DNA damage. Chromosomal DNA is shown as pairs of straight lines, Rad51 as open circles, and BRCA2 as grey bars. (a) Prevention of nonproductive DNA interactions. BRCA2–Rad51 interaction is proposed to suppress RAD51–BRCA2 interactions until DNA damage is present. When damage occurs, Rad51 is recruited to damaged sites where is polymerizes into nucleoprotein filaments. In this model, BRCA2 is not required for assembly of functional complexes at damaged sites, only to prevent a substantial fraction of Rad51 from being sequestered in a nonfunctional form. In a BRCA2-defective cell, mutant Rad51 becomes associated with DNA at random sites and is therefore not readily recruited to sites of damage. (b) Positive regulation. BRCA2 is proposed to be required for Rad51 to assemble into functional recombinational repair complexes at sites of damage. In BRCA-defective cells, Rad51 fails to associate with sites of damage due to lack of an assembly factor.