FEN1 also known as Rad2 is a member of the XPG/RAD2 endonuclease family and is one of ten proteins essential for cell-free DNA replication. FEN1 plays a crucial role in maintaining genome stability by participating in DNA replication, recombination, and repair processes. FEN1 is essential for the proper processing of Okazaki fragments during DNA replication and is involved in excising damaged DNA segments to correct mismatches during repair. Notably, FEN1 is highly homologous to the yeast RAD2 protein, indicating evolutionary conservation and importance in cellular functions. The C-terminal region of FEN1 interacts with proliferating cell nuclear antigen (PCNA), which enhances exonuclease activity during DNA replication. This interaction ensures efficient and accurate replication of DNA, thereby preventing genomic instability that can lead to various diseases, including cancer.