Isocodon effect on protein expression. (A) Isocodons for alanine mutation are shown in Lane 1-4 for GCG, GCA, GCC, GCU, respectively. Lane 5 is the wild type protein with leucine at the second position. (B) The expression using six serine isocodons are shown in Lane 1-6 for UCG, UCC, UCA, UCU, AGC, AGU respectively. (C) Expression of lysine using both isocodons. Lane 1 is the wild type leucine, Lanes 2 and 3 are AAA and AAG, respectively. (D) Ala, Ser, and Lys codon usage at the +2 position was compared between high expressing genes and randomly selected genes.