Virtual Rot analyses. (a) Virtual Rot analysis of hypothalamus library A. This plots the cumulative fraction of tags in the library against the total tags present for each tag-abundance level. The boundary between moderate- and low-abundance transcripts is set at the fraction of tags present only once in each library, as tag number (approximately 55,000) is not sufficient to comprehensively cover every cellular mRNA (around 300,000 molecules of mRNA/cell). (b) Virtual Rot analysis of adult retina library, carried out by the same method as (a). (c) mRNA abundance classes derived from the virtual Rot analysis in (a) and (b), compared to an actual Rot analysis of mouse whole-brain polysomal mRNA obtained from . ND, not determined.