Structural domains comprising this superfamily share the structure of the MepB protein of Staphylococcus aureus, encoded by the MepRAB operon. This operon has been shown to play a role in drug resistance and, although the functions of MepR and MapA are known, there is still little information regarding MepB's function. Structural studies have shown that this protein is structural similar to the PD-(D/E)XK family of endonucleases, although PD-(D/E)XK's catalytic active site residues are not conserved in MepB. MepB binds DNA and RNA, with a higher affinity for RNA and single stranded DNA than for double stranded DNA. The association of MepB with this drug resistance operon suggests that this protein is involved in antimicrobial response, role which is believed to be carried out through its interactions with nucleic acids PMID:24501097.