In parallel, whole chromosome tiling array interrogation of the RNA content of a variety of human tissues and cell lines revealed that, collectively, at least 93% of genomic bases are transcribed in one cell type or another 1,10-13. Since it is well established that highly expressed mRNAs dominate the non-ribosomal portion of the polyA+ transcriptome 7,8,10,14-19, normalization approaches were used to reduce the quantity of highly expressed transcripts in these cDNA analyses 7,8, and are implicit in tiling array approaches. ...any estimate of the pervasiveness of transcription requires inclusion of all data sources, and less than exhaustive analyses can only provide lower bounds for transcriptional complexity.