Inherited mutations in the XPD subunit of the general transcription/repair factor TFIIH yield the rare genetic disorder Xeroderma pigmentosum (XP), the phenotypes of which cannot be explained solely on the basis of a DNA repair defect. In cells derived from XP-D patients, we observed a reduction of the ligand-dependent transactivation mediated by several nuclear receptors (RARα, ERα, and AR). We demonstrate that the XPD mutation alters cdk7 function in RARα phosphorylation. Transactivation is restored upon overexpression of either the wild-type XPD or the RARαS77E (a mutation which mimics phosphorylated RARα). Thus, we demonstrate that the cdk7 kinase of TFIIH phosphorylates the nuclear receptor, then allowing ligand-dependent control of the activation of the hormone-responsive genes.