The FEN-1 C-terminus (orange) was docked onto the surface of the human PCNA trimer according to the crystal structure of PCNA with bound p21 peptide. Double-stranded DNA containing a short 5' flap was placed through the center of the PCNA trimer with the 5' end of the Okazaki fragment poised to be threaded through the FEN-1 helical clamp (green) and into the FEN-1 active site. With FEN-1 acting as an exonuclease and facing forward during DNA replication, the poximity of the enzyme to the growing flap would prevent the generation of long Okazaki fragments that could form FEN-1-resistant secondary structures. During DNA synthesis, the additional binding sites on the PCNA trimer could be occupied by additional replication factors.