Stop codon suppression or translational readthrough occurs when in translation a stop codon is interpreted as a sense codon, that is, when a (standard) amino acid is 'encoded' by the stop codon. Mutated tRNAs can be the cause of readthrough, but also certain nucleotide motifs close to the stop codon. Translational readthrough is very common in viruses and bacteria, and has also been found as a gene regulatory principle in humans, yeasts, bacteria and drosophila.   This kind of endogenous translational readthough constitutes a variation of the genetic code , because a stop codon codes for an amino acid. In the case of human malate dehydrogenase , the stop codon is read through with a frequency of about 4%.  Amino acids inserted at the stop codon depends of the identity of the stop codon itself: Gln, Tyr and Lys; have been found for UAA and UAG codon, while Cys, Trp, Arg for UGA codon have been identified by mass spectrometry.