Replicator fibers of composition F6S0 were prepared in borate buffer (25 mM in B2O3, pH 8.5) following a previously described procedure . Similarly, statistically distributed replicator fibers FnS6-n were prepared by mixing F and S building blocks (Figure ) in a 1:1 (v/v) ratio. In parallel, a 2.0 mM stock solution of precursors was prepared by mixing stock solutions of F (2.0 mM) and S (2.0 mM) in 1:1 (v/v) ratio, and adding 0.65 equivalents of NaBO3 (40 mM, in 25 mM B2O3 buffer) so that 65% of the thiols are oxidized. The oxidized stock solution of precursors was diluted to 0.20 mM by the same borate buffer and mixed with either 20 mol% F6S0 or 20 mol% FnS6-n fiber solutions, respectively, to a total volume of 200 µL. The replicator-precursor mixtures were incubated for 5 minutes and then centrifuged for 80 minutes at 15000 rpm (Eppendorf centrifuge model 5420). After centrifugation 40 µL of the supernatant was removed for analysis, and the rest of the aqueous phase was carefully discarded. The pellet was dispersed in 50 µL water and vortexed for 1 minute prior to analysis. Samples were analyzed using a Waters Acquity UPLC-H Class system equipped with a PDA detector. All analyses were performed using a reversed-phase UPLC column (Aeris Peptide 1.7 µm XB-C18 x 2.10 mm, Phenomenex). Further details are given in the SI section S8.