HDR begins with the recognition of a DSB. Enzymes such as MRN complex (MRE11, RAD50, and NBS1) detect the break and initiate the repair process. The damaged DNA ends are resected to produce single-stranded DNA (ssDNA). The ssDNA is then coated with RAD51 proteins, forming a nucleoprotein filament. This filament invades a homologous DNA sequence, usually a sister chromatid, to form a displacement loop (D-loop). DNA synthesis occurs using the homologous sequence as a template, followed by resolution of the D-loop and ligation of the newly synthesized DNA.