Repair of a DSB proceeds according to cell cycle stage. In G1, cells have a single copy of each chromosome (light blue and light green). If a break occurs in G1, the cell repairs the DSB by NHEJ, directly resealing the DNA ends (top). In G2, after chromosomes have been replicated, there is a sister chromatid, an identical copy of each original chromosome (dark blue and dark green, bottom). When a DSB occurs in G2, it is normally repaired by HR using either the sister chromatid or a homologous chromosome as a template. Repair from the sister (I) results in restoration of the exact information lost at the break site. Repair from the homolog (II), however, may lead to loss of heterozygosity if accompanied by a crossover.