The 5′‐3′ helicase activity of XPD is stimulated by p44, which directly interacts with XPD.32, 33 Notably, XPD mutations that are linked to developmental disorders XP and TTD inhibit the XPD‐p44 interaction and decrease XPD helicase and NER activity.33 This evidence concerns the gene ERCC2 and xeroderma pigmentosum.