(A) U2OS WT, RAD52 KO, and RAD52, 51AP1 DKO (clones C16 and C18) cells were synchronized in metaphase and telomeric FISH was performed using TelC-488 probe. (B) Signal free ends (SFEs) were quantified from (A). >25 metaphases were quantified from each cell line. Red lines: mean values. ****: P value <0.0001, **: P value 0.003. (C) Telomeres DNA content was quantified in U2OS WT, RAD52 KO, and RAD52, 51AP1 DKO (clones C16 and C18) cells after 1 month of passages. Error bars: SD, n=3 (experimental triplicates); ***: P value 0.001; **: P value 0.003 (D) Cell growth analysis of U2OS WT, RAD52 KO, and RAD52, 51AP1 DKO (clones C16 and C18) cells. (E) Representative images of β -galactosidase staining of U2OS WT, RAD52 KO, and RAD52, 51AP1 DKO (clones C16 and C18) cell populations. All cell lines were passaged in parallel for ~1 month.