Fig. 9. Model depicting competitive and cooperative relationships between components of the HR and NHEJ repair pathways in response to a site-specific DSB in fission yeast. Rhp55 promotes efficient use of HR in response to a site-specific DSB. In an rhp55Δ background, DSB repair involves both Ku70 and Rhp51, suggesting sequential use of NHEJ and HR pathways (dotted line). In the absence of the NHEJ pathway, elevated levels of Rhp51-dependent DSB repair are observed. DSB repair is associated additionally with chromosomal rearrangements in ku70Δ and lig4Δ mutants, indicating a potential role for the NHEJ pathway in suppressing crossovers. See text for details.