Oxacillin is a penicillinase-resistant β-lactam. It is similar to methicillin, and has replaced methicillin in clinical use. Another related compound is nafcillin. Since it is resistant to penicillinase enzymes, such as that produced by Staphylococcus aureus, it is widely used clinically in the US to treat penicillin-resistant Staphylococcus aureus. However, resistant strains called oxacillin-resistant Staphylococcus aureus (MRSA/ORSA) are highly prevalent in the U.S. and the U.K.