POLR1D and Treacher-Collins syndrome: In contrast to our understanding of the role of TCOF1 during embryogenesis and in the etiology and pathogenesis of TCS, there is a paucity of information about POLR1C and POLR1D. Therefore, we set out to explore the functional roles of polr1c and polr1d during embryogenesis and more specifically in craniofacial development in an effort to better understand the pathogenesis of TCS and the tissue-specificity of this ribosomopathy.