CodonW version 1.4.2 (John Peden, available at http://sourceforge.net/projects/codonw/), an integrated program, was utilized for calculating GC, GC3 contents and N c values and then carrying out correspondence analysis (CA), while GC, GC1, GC2 and GC3 contents were calculated by practical extraction and report language (PERL) scripts which were written by us. A3, T3, G3 and C3 contents, as well as RSCU and CAI values, were also calculated by using PERL scripts.