The human disease mutations discussed in the manuscript are shown in the context of the TFIIH structure. The areas depicted are: (i) The interaction interface between p8 and XPB, affected by a TTD mutation; (ii) the XPB N-terminal domain, affected by XP and TTD mutations; (iii) the active site region of XPD, affected mostly by XP and XP/CS mutations; (iv) the DNA-binding cleft of XPD, affected mostly by XP mutations; (v) the interaction site between XPD and p44, affected mostly by TTD mutations; (vi) the interaction site between MAT1 and XPD, affected by a TTD mutation (see text for further details).