Except for XP‐A (XPA‐defective), most XP patients do not exhibit a neurodegenerative phenotype; however, several cases with distinct mutations in ERCC3/XPB [30], ERCC2/XPD [31], or ERCC5/XPG [32] display the combined features of XP and CS (XPCS). Here, ERCC3 is linked to Cowden syndrome 1.