When analysing nucleotide synonymous and non-synonymous substitutions in a gene, which rate should I use dN/dS or dS/dN?

My hypothesis is that the gene I am working with has positive selection, but I am not quite sure which rate should I use dN/dS or dS/dN. Some softares calculate both and some just one of them. 

08 August 2015 1,049 2 View