Pre-rRNA processing in human cells. The major rRNA-processing pathways in human cells as initially derived from Hadjiolova et al²³  and modified by Rouquette et al.²⁴  Mature rRNA species are shown as filled boxes: 18S, ▪; 5.8S, ⊡; and 28S, ▨. External and internal transcribed sequences are shown as lines and are labeled above the primary transcript. Cleavage sites are designated with numbered and lettered arrows. Oligonucleotide probes used in Northern blot analysis are shown as lines below the primary transcript and are labeled with Greek letters. Two alternative pathways observed in human cells are shown below the 45S′ pre-rRNA that differ in the order of cleavages 1 and 2.