Genomic DNA is used as the template for both replication and transcription, 
whose machineries may collide and result in mutagenesis, among other damages. 
Because head-on collisions are more deleterious than codirectional collisions, 
genes should be preferentially encoded on the leading strand to avoid head-on 
collisions, as is observed in most bacterial genomes examined. However, why are 
there still lagging strand encoded genes? Paul et al. recently proposed that 
these genes take advantage of the increased mutagenesis resulting from head-on 
collisions and are thus adaptively encoded on the lagging strand. We show that 
the evidence they provided is invalid and that the existence of lagging strand 
encoded genes is explainable by a balance between deleterious mutations that 
bring genes from the leading to the lagging strand and purifying selection 
purging such mutants. Therefore, the adaptive hypothesis is neither 
theoretically needed nor empirically supported.