hidden stops. However, this hypothesis could not be validated with a larger data set. Stop-codons and hidden stops together are collectively referred as stop-signals. Researchers at University of Memphis found that the ratios of the stop-signals on the three reading frames of a genome (referred to as translation stop-signals ratio or TSSR) of genetically related bacteria, despite their great differences in gene contents, are much alike. This nearly identical genomic-TSSR value of genetically related bacteria may suggest that bacterial genome expansion is limited by their unique stop-signals bias of that bacterial species. Translational readthrough 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 readthrough 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%. The amino acid inserted at the stop codon depends on the identity of the stop codon itself: Gln, Tyr, and Lys have been found for the UAA and UAG codons, while Cys, Trp, and Arg for the UGA codon have been identified by mass spectrometry. Extent of readthrough in mammals have widely variable extents, and can broadly diversify the proteome and affect cancer progression. Use as a watermark In 2010 when Craig Venter unveiled the first fully functioning, reproducing cell controlled by synthetic DNA he described how his team used frequent stop codons to create watermarks in RNA and DNA to help confirm the results were indeed synthetic (and not contaminated or otherwise), using it to encode authors' names and website addresses. See also Genetic code Start codon Terminator gene References Molecular genetics Gene expression Protein biosynthesis