POLR1D and Treacher-Collins syndrome: Treacher Collins syndrome (TCS) is caused by mutations in genes involved in rRNA transcription, such as the Treacher Collins-Franceschetti syndrome 1 (TCOF1) and POLR1D and POLR1C genes, which encode subunits of RNA polymerase I and III (PolI/III; Box 1) (Dauwerse et al., 2011; Valdez et al., 2004).