Binding of XPC complex to the damaged site on the DNA substrate is followed by XPA and RPA recruitment. XPA is a metalloprotein that binds to different types of DNA damage. Binding of RPA, or Replicative protein A, a heterotrimer, initiates the formation of incision complex. This includes a serial assembly of the rest of the NER machinery consisting of TFIIH, XPG etc. Action of helicases in TFIIH complex opens up a bubble structure in the DNA template, exposing a fragment of DNA with lesion for endonuclease activity.