Data from [http://www.kazusa.or.jp](http://www.kazusa.or.jp).

More informations are available [here](http://www.kazusa.or.jp/codon/readme_codon.html
) and here is the original paper to cite:

```
Codon usage tabulated from the international DNA sequence databases:
status for the year 2000.
Nakamura, Y., Gojobori, T. and Ikemura, T. (2000) Nucl. Acids Res. 28, 292.
```
