Flap Endonuclease 1 (FEN1) is a member of the XPG/RAD2 endonuclease family. During DNA replication, FEN1 cleaves the 5'-overhanging flap structure and processes the 5' ends of Okazaki fragments for synthesis. FEN1 also exhibits RNase H activity by possessing 5'-3' exonuclease activity on gapped double-stranded or nicked DNA, FEN1 is involved in the long patch base excision repair (LP-BER) pathway, it can cleave within the apurinic/apyrimidinic (AP) site-terminated flap. FEN1 can prevent flaps from equilibrating into structures that lead to duplications and deletions. FEN1 is also involved in replication and repair of rDNA and in repairing mitochondrial DNA.