Cartoon overview of SSU processome and nuclear pre-40S assembly. (A) Schematic of the pre-rRNA encoding 18S, 5.8S, and 25S rRNAs. (B) Numbers 1–2 represent two potential QC steps: (1) balancing 40S:60S subunit production and (2) maturation of the ribosome neck. Described in more detail in the section titled “Nucleolar and nuclear pre-40S assembly.”