A) Comparison of USP1WT and deletion mutants (+UAF1) for activity on DNA-loaded PCNA-Ub (solid lines) and free PCNA-Ub (dashed lines) shows no increase in USP1ΔL1 activity on the loaded substrate. Left panel: Quantification of gel-based activity assays showing percentage of cleaved PCNA-Ub at the mentioned time points (n=2, biological replicates), Right panel: Quantification of the activation fold observed in USP1WT and mutants on DNA-loaded PCNA-Ub versus free PCNA-Ub (n=2, biological replicates). 