Many times expression does not occur in E. coli due to a difference in codons present in gene of interest and preferential codon use by the host E.coli strain. To solve this codon bias in host cell machinery, a few companies provide modified E. coli hosts that have extra tRNA coding genes (AUA, AGG, AGA, CUA, CCC and GGA) compensating for the scarcity of rare tRNA (Fig. 1).