Mutations in the human XPD gene can also cause trichothiodystrophy (TTD) and occasionally Cockayne syndrome (CS), which is seen only in combination with XP (XP/CS) in XP-D patients (Fig. 1A; supplementary material Table S1; Fan et al., 2008; Lehmann, 2001). The gene discussed is ERCC2; the disease is trichothiodystrophy.