POLR1D and Treacher-Collins syndrome: The literature describes four clinical TCS subtypes: TCS1 (OMIM 154500) is a result of mutations in the TCOF1 gene (OMIM 606847), TCS2 (OMIM 613717) is caused by mutations in the POLR1D gene (OMIM 613715), TCS3 (OMIM 248390) is caused by POLR1C gene mutations (OMIM 610060), and TCS4 (OMIM 618939) is caused by mutations in the POLR1B gene (OMIM 602000).