A novel ArsR regulator of a bacterial arsenical resistance (ars) operon is selective for methylarsenite (MAs(III)) over As(III). This surface model of the CadC aporepressor structure shows the location of metal(loid) binding sites. The S3 As(III)‐binding site of AfArsR (purple) and the S2 MAs(III)‐binding site of SpArsR (blue) differ by a single cysteine residue. For details see the article by Chen et al . on pp. 469–478 of this issue.