ERCC2 and xeroderma pigmentosum: The N-terminus of XPD forms a short two-stranded β-sheet near the nucleotide-binding site in XPD RecA1 (which is empty in our structure) and may contribute to the stabilization of the bound nucleotide via the aromatic side chains of Y14 and Y18, the latter being affected by the Y18H mutation in an XP/TTD patient (Kralund et al., 2013) (Figure 5—figure supplement 1A–C).