Base excision repair (BER) is a cellular mechanism, in [[DNA repair]] throughout the [[cell cycle]]. It is responsible primarily for removing small, non-helix-distorting base lesions from the [[genome]]. The related [[nucleotide]] excision repair pathway repairs bulky helix-distorting lesions. BER is important for removing damaged bases that could otherwise cause [[mutation]]s by mispairing or lead to breaks during [[DNA replication]]. BER is initiated by DNA glycosylases, which recognize and remove specific damaged or inappropriate bases, forming AP sites. These are then cleaved by an AP endonuclease. The resulting single-strand break can then be processed by either short-patch (where a single nucleotide is replaced) or long-patch BER (where 2–10 new nucleotides are synthesized).