EXO1 exonucleolytically degrades the strand being repaired in a 5' to 3' direction (Zhang et al. 2005, Orans et al. 2011, and inferred from EXO1 activity with MSH2:MSH6) to create a single -stranded gap extending 90-170 nucleotides beyond the insertion/deletion loop (IDL) (Fang and Modrich 1993). MLH1:PMS2 limits the length of excisions by EXO1 (Zhang et al. 2005). EXO1 also forms a complex with PCNA during S phase (Liberti et al. 2011). RPA binds the resulting single-stranded DNA (Lin et al. 1998, Ramilo et al. 2002, Zhang et al. 2005, reviewed in Iftode et al. 1999).