Nucleic Acids Research, 2011, Vol. 39, No. 1 281 Figure 1. Pre-rRNA processing in HeLa cells. (A) Comparison between the yeast and human large polycistronic rRNA precursor. These two pre-rRNAs have been scale-drawn. (B) 18S rRNA-processing pathways in Human (left panel) and yeast (right panel). Two pathways are presented, according to the kinetics of the ETS1/50-ETS or ITS1 maturation. The steps at which Enp1p and Tsr1p are involved are indicated. (C) Processing pathways for the large ribosomal subunit RNAs in human (left panel) and yeast (right panel). the 30-end of the 18S rRNA (20). Enp1p and Bms1p are known to be implicated in the processing of ETS1 and ITS1 (21–23), but their molecular functions remain unclear. Rrp12p, a HEAT repeat containing protein, is able to interact with nuclear pore complex components and appears to be required for the nuclear export of the pre-40S particle (24). These proteins are joined by other pre-ribosomal factors that assemble speciﬁcally with the pre-40S particle. Hence, Ltv1p in yeast, as well as the kinase hRio2 in human, were proposed as adapters for exportin Crm1p which is required for transport to the cytoplasm (25,26). Hrr25p, a protein kinase, regulates as- sociation of the ribosomal protein Rps3p to the pre-40S particle, allowing a structural modiﬁcation of the pre-40S particle beak, which could be required for passage through the nuclear pore complex (27). The Rio kinases and Tsr1p