Various researchers have attempted to measure selection acting on overlapping reading frames, by investigating the K a /K s ratio within these regions for seperate reading frames [4, 7, [14] [15] [16] . Comparing non-synonymous to synonymous substitution rates only makes sense when the synonymous substitutions are unconstrained. In the case of coding for multiple genes, however, a synonymous substitution in one gene may well be non-synonymous in the other and thus constrained. This biases the analysis towards an under-estimation of the 'true' synonymous substitution rate and thus can lead to the false inference of positive selection.