Schematic representation of the human (top) and Tetraodon(bottom) rRNA gene organization. In humans, a 30-kb nontranscribed spacer (open boxes, partially represented) separates a tandem repetition of the 18S, 5.8S, and 28S genes (black boxes) interspersed by intergenic transcribed spacers and external transcribed spacers (ITSs and ETSs, respectively; horizontal lines). In the 8303-bpTetraodon sequence, the positions of the first and last base of each gene are indicated based on their homology with the human sequence. The sequence and position of the Tetraodonnontranscribed spacer is unknown. A multiple alignment of theTetraodon sequence is available under EMBL accession number DS42722.