NER factors and gene transcription. Upon gene activation, the formation of preinitiation complex (PIC) precedes the recruitment of XPC, which allows the sequential arrival of the other NER factors in the following order CSB, XPA/RPA, XPG/ERCC1-XPF, and Gadd45. The association of NER factors and Gadd45 with the transcription machinery leads to a cascade of histone PTMs. Concomitantly, an active demethylation of CpG islands occurs.