There are 64 (43) possible codons that code for 20 amino acids (and stop signals) so one amino acid may be encoded by several codons (e.g., serine is encoded by six codons in nuclear genes). It is therefore interesting to know the codon usage for each amino acid. In MEGA, the numbers of the 64 codons used in a gene can be computed either for one specific sequence or for all examined sequences. In addition to the codon frequencies, MEGA also writes the Sharp et al. (1986) relative synonymous codon usage (RSCU) statistic (see Nei and Kumar 2000, page 11).