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One of the most versatile mammalian DNA repair pathways is nucleotide excision repair (NER) in which repair factors assemble sequentially on DNA damage. First, XPC-hHR23b locates DNA injuries in a critical step initiating the formation of the preincision complex (PInC). Subsequently, XPC-hHR23b recruits the transcription/repair factor TFIIH (Sugasawa et al., 1998) containing ten subunits (XPB, XPD, p62, p52, p44, p34, cdk7, cyclH, MAT1, and TTDA; Compe and Egly, 2012). Some of them are not always strictly part of the complex (Zhovmer et al., 2010). For instance, TTDA is more strongly associated with TFIIH after induction of NER-specific DNA lesions (GigliaMari et al., 2006). Loading of TFIIH promotes the unwinding of the damaged DNA that provides a three-dimensional structure able to recruit XPA and RPA (Oksenych et al., 2009). The edges of the DNA bubble created by the TFIIH helicases are recognized by the two junction-specific endonucleases XPG and ERCC1-XPF. Altogether, XPC, TFIIH, XPA, RPA, XPG, and ERCC1-XPF form the PInC that generates 3 and 5 single DNA incisions relative to the damage (O’Donovan et al., 1994; Sijbers et al., 1996). Excision of a 24–32-mer damaged oligonucleotide precedes the gap-filling DNA resynthesis step (Ogi et al., 2010). Although XPC has a high affinity for several NER substrates such as the UV light–induced pyrimidine– pyrimidone (6–4) photoproducts, its binding to some of them can be weak and require the DDB1–DDB2(XPE) complex (Tang et al., 2000). Mutations in genes coding for XPA, XPB, XPC, XPD, DDB2, XPF, XPG, and TTDA give rise to either xeroderma pigmentosum (XP), trichothiodystrophy (TTD), or a combined XP and Cockayne syndrome. These inherited human disorders exhibit a broad spectrum of clinical features including a common photosensitivity of the skin (Lehmann, 2003). In the current NER model, the recognition of the DNA lesion by XPC-hHR23b nucleates the formation of the PInC (Sugasawa et al., 1998). However, the participation of NER factors in other cellular processes that do not involve DNA lesions such as transcription (Fong et al., 2013) raised the question of the role of DNA injuries in the formation of the PInC. To obtain molecular insights into the assembly of the NER factors, we analyzed the formation of the PInC independently of the presence of DNA damage using the lactose operator (LacO)/lactose repressor (LacR) reporter system that In nucleotide excision repair (NER), damage recognition by XPC-hHR23b is described as a critical step in the formation of the preincision complex (PInC) further composed of TFIIH, XPA, RPA, XPG, and ERCC1XPF. To obtain new molecular insights into the assembly of the PInC, we analyzed its formation independently of DNA damage by using the lactose operator/repressor reporter system. We observed a sequential and ordered self-assembly of the PInC operating upon immobilization of individual NER factors on undamaged chromatin and mimicking that functioning on a bona fide NER substrate. We also revealed that the recruitment of the TFIIH subunit TTDA, involved in trichothiodystrophy group A disorder (TTD-A), was key in the completion of the PInC. TTDA recruits XPA through its first 15 amino acids, depleted in some TTD-A patients. More generally, these results show that proteins forming large nuclear complexes can be recruited sequentially on chromatin in the absence of their natural DNA target and with no reciprocity in their recruitment. Sequential and ordered assembly of a large DNA repair complex on undamaged chromatin