A silent mutation is a type of mutation that occurs in DNA and does not have an observable effect on the organisms phenotype. It is a specific type of neutral mutation that can be produced by insertions or deletions, which cause a shift in the reading frame. Silent mutations lead to a change of one of the letters in the triplet code that represents a codon, but despite the single base change, the amino acid that is coded for remains unchanged or similar in biochemical properties. Silent mutations are often treated as though they are evolutionarily neutral because they do not alter protein function. However, many organisms exhibit codon usage biases, suggesting that there is selection for the use of particular codons due to the need for translational stability. Transfer RNA (tRNA) availability is one of the reasons that silent mutations might not be as silent as conventionally believed. In molecular cloning experiments, it can be useful to introduce silent mutations into a gene of interest in order to create or remove recognition sites for restriction enzymes. Silent mutations can also cause problems with exonic splicing enhancers (ESEs), resulting in changes in mRNA processing of the genetic information. Recent studies have shown that synonymous mutations can influence the amount of protein that is produced, and optimal codons are faster for cells to process and lead to increased protein production.