Degeneracy

0-fold degenerate sites are those at which all changes are nonsynonymousRH_Nonsynonymous_change.

2-fold degenerate sites are those at which one out of three changes is synonymousRH_Synonymous_change.  (All sites at which two out of three changes are synonymous also are included in this category.)

4-fold degenerate sites are those at which all changes are synonymous.