Many amino acids are coded by more than one codon; thus multiple codons for a given amino acid are synonymous. However, many genes display a non-random usage of synonymous codons for specific amino acids. A measure of the extent of this non-randomness is given by the Relative Synonymous Codon Usage (RSCU) (Sharp et al. 1986).
The RSCU for a particular codon (i) is given by
RSCUi = Xi / مم XI /n
where Xi is the number of times the ith codon has been used for a given amino acid, and n is the number of synonymous codons for that amino acid.