Effective-number-of-codons as a function of cDNA position, relative to the start of the coding region, and averaged over all cDNAs with a 51-bp sliding window. Shown here are O. sativa, Z. mays, A. thaliana, and N. tabacum. Ne is a statistical measure of codon bias. It is 20 if only one codon is likely to be used for each amino acid, implying maximal bias. It is 61 if every codon is equally likely to be used, implying minimal bias.