The mammalian nucleotide excision repair (NER) pathway removes dangerous bulky adducts from genomic DNA. Failure to eliminate these lesions can lead to oncogenesis, developmental abnormalities and accelerated ageing. TFIIH is a central NER factor that opens the damaged DNA through the action of its two helicases (XPB and XPD) prior to incision. Here we review our recently published data that suggest specific and distinct roles for these two helicases in NER. We also discuss the regulation of XPB and XPD enzymatic activities within TFIIH and repair complexes, and show that mutations impeding enzyme-regulator interaction contribute to genetic disorders. Understanding the fundamental molecular mechanism regulating NER is a crucial aspect of cancer therapy since the resistance to chemotherapy treatment relies on the capacities of the cell to eliminate drug-induced DNA lesions.