The IUPAC has designated the symbols for nucleotides. Apart from the five (A, G, C, T/U) bases, often degenerate bases are used especially for designing PCR primers. These nucleotide codes are listed here.  Some primer sequences may also include the character "I", which codes for the non-standard nucleotide inosine.  Inosine occurs in tRNAs and will pair with adenine, cytosine, or thymine.  This character does not appear in the following table, however, because it does not represent a degeneracy.  While inosine can serve a similar function as the degeneracy "D", it is an actual nucleotide, rather than a representation of a mix of nucleotides that covers each possible pairing needed.