In PLoS Biology this week, investigators at the University of Toronto report that most "dark matter" transcripts are associated with known genes in the mouse and human genomes. Using a combination of RNA-seq information and data obtained from tiling arrays experiments, the team shows that most of the seqfrags — or transcribed regions outside known exons and non-coding RNAs — are within introns, "raising the possibility that they are fragments of pre-mRNAs," they write.