The plot of optimized elements’ size against transgene expression efficiency of each expression cassette. A. Common scheme of the expression cassette generation in this study. The target element sites varied for optimization are indicated. B. The normalized EGFP expression level by each cassette plotted against the size of each expression cassette with modification in WPRE and bGHpA sites. Note that size reduction of about 399 bp in these two elements in CW3SL still gives similar level of transgene expression as that of CWB.