Expressionsniveau

Expressionsniveau is a term used to describe the quantitative level at which a gene, transcript, or protein is expressed in a given biological sample. It aims to reflect the abundance of the RNA or protein and hence the activity of the underlying gene in a particular tissue, cell type, or environmental condition. In practice, the concept is most commonly encountered in transcriptomics and proteomics, where expression levels are measured and compared across samples.

In transcriptomic workflows, expressionsniveau is typically derived from sequencing or hybridization data. RNA sequencing (RNA-Seq) yields read counts for each gene, which are then normalized to enable comparisons across samples using metrics such as TPM (transcripts per million), RPKM/FPKM (reads/fragments per kilobase per million), or CPM (counts per million). Data are often transformed on a log scale to stabilize variance and improve interpretability. In proteomics, expression levels may be inferred from peptide intensities or spectral counts and similarly normalized.

Expressionsniveau is central to differential expression analysis, where researchers identify genes whose expression differs significantly between conditions, treatments, or phenotypes. Such analyses support the discovery of regulatory networks, pathway activity, and potential biomarkers. It is also used to assess tissue specificity, developmental regulation, and responses to environmental stimuli.

Limitations and caveats include that mRNA abundance does not always mirror protein levels due to post-transcriptional regulation, translation efficiency, and protein turnover. Technical biases, batch effects, sample quality, and the choice of normalization method can influence estimates. Single-cell approaches add complexity by revealing cell-to-cell variability in expressionsniveau.