GTF2H5 and trichothiodystrophy: In addition to these XPCS mutations, specific pathogenic variants in XPB, XPD, and the p8 subunit of TFIIH (TTDA, encoded by GTF2H5) can cause trichothiodystrophy (TTD) [33], which has similarities with CS but also includes sulphur‐deficient brittle hair [34].