Putative biochemical function for DKC1, TCOF1, SBDS, RPS19, and RMRP in ribosomal biogenesis. For simplicity, the mammalian rRNA processing pathway is depicted, whereas the putative function of each gene product is deduced from data obtained from yeast and human orthologs. In the mammalian rRNA processing pathway, cleavages at the external transcribed sequence (5′ETS) and internal transcribed sequences (ITS1 and ITS2) lead to maturation of 18S, 5.8S, and 28S rRNA species. The 18S rRNA is a component of the 40S ribosomal subunit. The 5.8S and 28S rRNAs, together with the 5S rRNA (not shown), are components of the 60S ribosomal subunit. The 40S and 60S subunits are assembled into 80S ribosomes. Illustration by Frank Forney.