design: USER-S1-END-seq protocol was used with a spike-in control (20% Lig4/ pre-B cells with a zinc-finger-induced DSB at the TCR enhancer mixed with Ramos cells) for normalization. Ramos cells (30 million) were embedded in agarose plugs, treated with Proteinase K, washed, and subjected to USER enzyme and S1 nuclease treatments for DNA end processing. DNA ends were blunted with exonuclease VII and exonuclease T, followed by adapter ligation, sonication, and library preparation.