The targets of HDR (Homology-Directed Repair) primarily include DNA break sites. This repair mechanism involves key proteins such as RAD51, BRCA1, BRCA2, among others. These proteins form a complex repair complex within the nucleus, utilizing a homologous DNA template to precisely repair breaks. HDR is particularly active during the S and G2 phases of the cell cycle, crucial for maintaining genomic stability. The expression of these key proteins shows distinct specificity in different cells and tissues, and mutations in their genes (such as BRCA1/2 mutations) can lead to a loss of repair function, increasing the risk of certain cancers.