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Fig. 4 Analysis of ribosome synthesis show defects in rRNA maturation. (A) Total RNAs extracted from LCLs of DBA cases (gray) or unaffected individuals (black) were analyzed by Northern blot with probes ITS2, 5?-ITS1, 18S, and 28S (positions of the probes in panel C). The ratios of 28S to 18S rRNAs were quantified, normalized to the mean value obtained for controls, and are displayed as log values. (B) Detection of the cryptic pre-rRNA species 36S, 36S-C, and 32.5S with the ITS1-5.8S probe. The intensity profiles shown for the ITS1-5.8S probe were normalized relative to the levels of 28S rRNA. (C) Schematic representation of the pre-rRNAs derived from the 47S primary ribosomal transcript in human cells by endonucleolytic cleavage (horizontal lines) and exonucleolytic processing (Pacman). Impairment of cleavage at site 2 leads to accumulation of the 36S and 36S-C cryptic precursors by direct ITS1 cleavage at site E. Domain C corresponds to a highly conserved domain in ITS1 that blocks exonuclease progression. The positions of the Northern blot probes are indicated below the 47S pre-rRNA and their sequences are listed in Supplemental Table 8.