The study by Ramsköld et al. (2009) investigates the transcriptome composition and complexity in various human and mouse tissues and cell lines using RNA-Seq data. The authors found that approximately 8,000 protein-coding genes are ubiquitously expressed, contributing to about 75% of all mRNAs in most tissues. These genes encode proteins that are often intracellular and involved in metabolism, transcription, RNA processing, or translation. In contrast, genes for secreted or plasma membrane proteins are generally expressed in only a subset of tissues. The distribution of expression levels was broad but continuous, with no distinct expression classes of genes. Removing 3' UTR reads from expression estimates improved correlation with qRT-PCR data. Muscle and liver had simpler transcriptomes, with a large fraction of transcripts coming from a few highly expressed genes, while brain, kidney, and testis had more complex transcriptomes with a larger number of genes expressed and less dominance from a few highly expressed genes. The study also found that brain mRNAs had longer 3' UTRs on average compared to liver and muscle mRNAs, and that transcripts involved in development, morphogenesis, and signal transduction had longer UTRs. The results support a model where variable exterior components feed into a large, densely connected core of intracellular proteins.The study by Ramsköld et al. (2009) investigates the transcriptome composition and complexity in various human and mouse tissues and cell lines using RNA-Seq data. The authors found that approximately 8,000 protein-coding genes are ubiquitously expressed, contributing to about 75% of all mRNAs in most tissues. These genes encode proteins that are often intracellular and involved in metabolism, transcription, RNA processing, or translation. In contrast, genes for secreted or plasma membrane proteins are generally expressed in only a subset of tissues. The distribution of expression levels was broad but continuous, with no distinct expression classes of genes. Removing 3' UTR reads from expression estimates improved correlation with qRT-PCR data. Muscle and liver had simpler transcriptomes, with a large fraction of transcripts coming from a few highly expressed genes, while brain, kidney, and testis had more complex transcriptomes with a larger number of genes expressed and less dominance from a few highly expressed genes. The study also found that brain mRNAs had longer 3' UTRs on average compared to liver and muscle mRNAs, and that transcripts involved in development, morphogenesis, and signal transduction had longer UTRs. The results support a model where variable exterior components feed into a large, densely connected core of intracellular proteins.