This review summarizes the current understanding of the global relationships between protein and mRNA expression levels, and the correlation between these parameters. It discusses how the comparison of protein and mRNA expression levels can provide insights into the regulation of gene expression. The article highlights the importance of translation and protein degradation in determining the number of protein molecules per mRNA, and how these processes are regulated at both the global and gene-specific levels. Translation in eukaryotic cells involves initiation, elongation, and termination, and is influenced by factors such as the Kozak sequence, ribosome binding to upstream open reading frames (uORFs), and secondary structures in the mRNA. Translation elongation is affected by the availability of tRNAs and the choice of codons. The poly(A) tail length of mRNA also influences translation efficiency. Translation can be regulated by factors such as phosphorylation, proteolysis, and cis-regulatory elements like RNA-binding proteins and microRNAs. Protein degradation is highly specific and regulated, involving two major systems: lysosomal degradation and ubiquitin-mediated proteolysis. Degradation signals, such as degrons, are encoded in the protein sequence and are recognized by specific enzymes. The ubiquitin pathway is conserved in all eukaryotes, while prokaryotes lack ubiquitin but have homologs. Large-scale methods have been developed to study protein translation and turnover, including microarrays, RIP-Chip, TRAP, and sucrose gradients. These methods allow for the measurement of translation efficiency and protein degradation rates. Absolute concentrations of proteins and mRNAs can be estimated using various techniques, including Western blotting, mass spectrometry, and next-generation sequencing. The protein-per-mRNA ratio is a key factor in understanding the regulation of gene expression. It reflects the combined outcomes of translation and protein degradation, and is influenced by sequence characteristics. In bacteria, codon usage and amino acid composition are strong correlates of the protein-per-mRNA ratio. In yeast, codon usage, sequence around the translation initiation site, and tRNA adaptation are also correlated with the ratio. In multi-cellular organisms, the correlation is less strong, but still significant. The review concludes that the relationship between protein and mRNA concentrations is a simple measure that informs about the combined outcomes of complex processes. The protein-per-mRNA ratio is influenced by the accuracy of measurements and the significance of observed correlations. The study highlights the importance of translation and protein degradation in gene expression regulation, and the need for further research to understand the regulatory mechanisms in different organisms.This review summarizes the current understanding of the global relationships between protein and mRNA expression levels, and the correlation between these parameters. It discusses how the comparison of protein and mRNA expression levels can provide insights into the regulation of gene expression. The article highlights the importance of translation and protein degradation in determining the number of protein molecules per mRNA, and how these processes are regulated at both the global and gene-specific levels. Translation in eukaryotic cells involves initiation, elongation, and termination, and is influenced by factors such as the Kozak sequence, ribosome binding to upstream open reading frames (uORFs), and secondary structures in the mRNA. Translation elongation is affected by the availability of tRNAs and the choice of codons. The poly(A) tail length of mRNA also influences translation efficiency. Translation can be regulated by factors such as phosphorylation, proteolysis, and cis-regulatory elements like RNA-binding proteins and microRNAs. Protein degradation is highly specific and regulated, involving two major systems: lysosomal degradation and ubiquitin-mediated proteolysis. Degradation signals, such as degrons, are encoded in the protein sequence and are recognized by specific enzymes. The ubiquitin pathway is conserved in all eukaryotes, while prokaryotes lack ubiquitin but have homologs. Large-scale methods have been developed to study protein translation and turnover, including microarrays, RIP-Chip, TRAP, and sucrose gradients. These methods allow for the measurement of translation efficiency and protein degradation rates. Absolute concentrations of proteins and mRNAs can be estimated using various techniques, including Western blotting, mass spectrometry, and next-generation sequencing. The protein-per-mRNA ratio is a key factor in understanding the regulation of gene expression. It reflects the combined outcomes of translation and protein degradation, and is influenced by sequence characteristics. In bacteria, codon usage and amino acid composition are strong correlates of the protein-per-mRNA ratio. In yeast, codon usage, sequence around the translation initiation site, and tRNA adaptation are also correlated with the ratio. In multi-cellular organisms, the correlation is less strong, but still significant. The review concludes that the relationship between protein and mRNA concentrations is a simple measure that informs about the combined outcomes of complex processes. The protein-per-mRNA ratio is influenced by the accuracy of measurements and the significance of observed correlations. The study highlights the importance of translation and protein degradation in gene expression regulation, and the need for further research to understand the regulatory mechanisms in different organisms.