When the polymerase delta:PCNA complex reaches a downstream Okazaki fragment, strand displacement synthesis occurs. The primer containing 5'-terminus of the downstream Okazaki fragment is folded into a single-stranded flap (Podust et al. 1995, Bae et al. 2001, Maga et al. 2001). The helicase activity of either WRN (Werner syndrome protein) or BLM (Bloom syndrome helicase) is needed for DNA polymerase delta progression and strand displacement synthesis across G-rich telomeric repeats during lagging strand (C-strand) synthesis (Li et al. 2017).