With the introduction of methods of rapid nucleic acid sequence determination, synthesis of mixed oligonucleotide probes and computer-assisted analysis of nucleic acid sequences, the use of a single symbol to designate a variety of possible nucleotides at a single position has become widespread over the last few years. Whereas the use of, for example, the symbols R and Y to designate purine (A or G) and pyrimidine(C or T) ribonucleotides respectively [1] is generally accepted, no agreed symbols exist for the other possible combinations. Indeed, a plethora of diverse systems have proliferated in the last few years(2-11]. It is striking that, in one extreme case, the combination (C or G) has been represented by at least five different symbols [2, 3, 4, 8, 11]. A standardized set of symbols is thus required to prevent confusion.