In mammals, a second type of BER is observed, which is often used preferentially during ATP shortage – long patch BER. Instead of just removing the single damaged base, long-patch BER repairs a patch several nucleotides long. To achieve this, a different DNA polymerase, δ/ɛ adds several nucleotides that displace the original nucleotides. This results in an overhang of oligonucleotides called a flap, which contains the damaged base. In the presence of a replication factor called proliferating cell nuclear antigen or PCNA, a special endonuclease called Flap Endonuclease (FEN) removes this flap before a DNA ligase seals the nick. The mechanism of long patch BER is particularly useful to repair damages resulting from ionizing radiation.