CS and Cowden syndrome 1: If so, it is important to determine if the defective TFIIH-XPG interaction is the underlying cause of CS due to mutations in CSA, CSB, XPB and XPD. Core-TFIIH and CAK are well associated in CSA, CSB, and XPB/CS cells, and the failure of each mutant gene to interact with the core-TFIIH complex might not cause CS (Ito et al., 2007).