 Most notably, the UAG codon appears to act as a stop codon in many of that organism's proteins, with only a single established use in coding pyrrolysine in that organism.By contrast, in methanogenic archaea it was not possible to identify any unambiguous UAG stop signal.Because there was only one known site where pyrrolysine is added in "D. hafniense" it was not possible to determine whether some additional sequence feature, analogous to the SECIS element for selenocysteine incorporation, might control when pyrrolysine is added.