The sequences of the oligonucleotides were listed in Supplementary Table S1. HPLC-purified fluorescent labeled primers, biotinylated extension oligonucleotides, and splint oligonucleotides were purchased from Integrated DNA Technologies (Coralville, IA, USA). Single-stranded oligonucleotides with ICL precursors, 39mer 20-bp ICL that contains two franking uracil bases around the crosslinked base (39+20(6)-ICL), and 39mer 6-bp ICL (39+6-ICL) were synthesized and purified as previously reported (37 (link)). The identity of the ICLs was confirmed by LC–MS (Supplementary Figure S1A).