ERCC2 and Cowden syndrome 1: These experiments indicated that TFIIH might act as an elongation factor that productively interacts with RNA Pol I. The mutated XPB and XPD subunits of TFIIH, as found in CS cells, reduced the affinity of TFIIH for rDNA and decreased the productivity of the transcription by RNA Pol I, implying qualitative dysfunction of TFIIH in the XPB- and XPD-mutated CS cells.