We also observe evidence for selection on molybdenumrelated genes in our d N /d S data. On the branch leading to the pathogenic Mycobacteria, several orthogroups with high log likelihood scores when testing for selection are related to molybdenum (see Supplementary Information website). The orthogroup containing BisC (biotin sulfoxide reductase, a molybdoenzyme), as well as the orthogroup containing ModA (an ABC-family molybdate transporter), are among those with the highest d N /d S values on the branch leading to the pathogens. MoaB2 is one of the highest-scoring genes on all three branches tested.