Frequency of occurrence for amino acids alanine, leucine, glycine, and serine, as 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. When all 20 amino acids occur with equal probability, the normalized frequencies are 1.