Codon bias, or the preferential use of certain codons, has significant consequences for cellular processes and is central to fields like molecular evolution and biotechnology. Despite the genetic code's redundancy, synonymous mutations (those that do not change the amino acid sequence) do not always have neutral effects. Codon-usage bias (CUB) is a key factor in shaping gene expression and cellular function, influencing processes such as RNA processing, protein translation, and folding. CUB is widespread and can be very strong, with some species avoiding certain codons entirely. In applied settings, specific codons can increase transgene expression by over 1,000-fold. Several hypotheses explain CUB, including mutational and selective mechanisms. Mutational explanations suggest that codon bias arises from underlying mutational processes, while selective explanations propose that synonymous mutations influence organismal fitness. These mechanisms are not mutually exclusive, and both play roles in shaping CUB across species and within genomes. CUB is influenced by factors such as tRNA abundance, gene expression levels, and mRNA structure. For example, highly expressed genes often show stronger CUB to match tRNA availability, enhancing translation efficiency or accuracy. Studies have shown that CUB varies across species, genomes, and genes, often due to selection. In mammals, CUB is influenced by GC content and isochores, while in other species, it is shaped by tRNA availability and gene expression. CUB also affects gene expression in specific contexts, such as splicing and mRNA stability. For instance, certain codons may be selected for to facilitate proper splicing or to avoid mistranslation. Recent advances in synthetic biology, mass spectrometry, and sequencing have enabled systematic studies of CUB's molecular and cellular consequences. These studies have refined our understanding of how initiation, elongation, degradation, and misfolding influence protein expression levels and cellular fitness. This information helps researchers distinguish the forces shaping natural CUB patterns and improve design principles for vaccine development and gene therapy. In heterologous gene expression, CUB can significantly affect protein yields. However, the relationship between codon adaptation and protein levels is complex, as overexpression can lead to amino acid starvation and changes in tRNA abundances. Strategies for optimizing heterologous expression now consider factors such as global nucleotide content, local mRNA folding, and codon pair bias, in addition to traditional tRNA abundance-based approaches. Overall, CUB is a critical factor in both natural and applied contexts, influencing gene expression, protein synthesis, and cellular function. Understanding the mechanisms and consequences of CUB is essential for advancing fields such as biotechnology and molecular biology.Codon bias, or the preferential use of certain codons, has significant consequences for cellular processes and is central to fields like molecular evolution and biotechnology. Despite the genetic code's redundancy, synonymous mutations (those that do not change the amino acid sequence) do not always have neutral effects. Codon-usage bias (CUB) is a key factor in shaping gene expression and cellular function, influencing processes such as RNA processing, protein translation, and folding. CUB is widespread and can be very strong, with some species avoiding certain codons entirely. In applied settings, specific codons can increase transgene expression by over 1,000-fold. Several hypotheses explain CUB, including mutational and selective mechanisms. Mutational explanations suggest that codon bias arises from underlying mutational processes, while selective explanations propose that synonymous mutations influence organismal fitness. These mechanisms are not mutually exclusive, and both play roles in shaping CUB across species and within genomes. CUB is influenced by factors such as tRNA abundance, gene expression levels, and mRNA structure. For example, highly expressed genes often show stronger CUB to match tRNA availability, enhancing translation efficiency or accuracy. Studies have shown that CUB varies across species, genomes, and genes, often due to selection. In mammals, CUB is influenced by GC content and isochores, while in other species, it is shaped by tRNA availability and gene expression. CUB also affects gene expression in specific contexts, such as splicing and mRNA stability. For instance, certain codons may be selected for to facilitate proper splicing or to avoid mistranslation. Recent advances in synthetic biology, mass spectrometry, and sequencing have enabled systematic studies of CUB's molecular and cellular consequences. These studies have refined our understanding of how initiation, elongation, degradation, and misfolding influence protein expression levels and cellular fitness. This information helps researchers distinguish the forces shaping natural CUB patterns and improve design principles for vaccine development and gene therapy. In heterologous gene expression, CUB can significantly affect protein yields. However, the relationship between codon adaptation and protein levels is complex, as overexpression can lead to amino acid starvation and changes in tRNA abundances. Strategies for optimizing heterologous expression now consider factors such as global nucleotide content, local mRNA folding, and codon pair bias, in addition to traditional tRNA abundance-based approaches. Overall, CUB is a critical factor in both natural and applied contexts, influencing gene expression, protein synthesis, and cellular function. Understanding the mechanisms and consequences of CUB is essential for advancing fields such as biotechnology and molecular biology.