Cells and the EJ2-GFP and MMEJ-GFP aNHEJ reporter cells were cultured as described (8 (link), 9 (link), 41 ). For inducing DNA damage, cells were treated with VP16 from Sigma Aldrich (Cat # E1383-25MG) and Cytosine B-d-arabinofuranoside (Ara-C) from Sigma Aldrich (Cat # C6645).