+ <p>
+ The use of X to represent an unknown base is acknowledged, but this is not recommended as the symbol refers to xanthine (see IUPAC-IUB Commission on Biochemical Nomenclature (CBN). <a href="https://iupac.qmul.ac.uk/misc/naabb.html">Abbreviations and Symbols for Nucleic Acids, Polynucleotides and their Constituents.</a>)
+ </p>