In previous works, we could show that cells from CS patients with CSA/CSB or TFIIH mutations (Assfalg et al. 2012; Bradsher et al. 2002; Koch et al. 2014) are characterized by severe disturbances in RNA polymerase I transcription, the key step of ribosomal biogenesis. This evidence concerns the gene ERCC3 and Cowden syndrome 1.