ERCC3 and Cowden syndrome 1: Given the implication of XPB NTD in human disease, we tested three XPB NTD mutants that include a complete NTD deletion (XPB320–782), a substitution (XPBF99S) expressed in XP-B/CS patient-derived cells (XPCS1BA = XP-B/CSF99S) and a substitution (XPBT119P) expressed in TTD patient-derived cells (TTD6VI = TTD-XPBT119P).