Resistance to methicillin is mediated via the "mec" operon, part of the staphylococcal cassette chromosome mec (SCC"mec"). SCCmec is a family of mobile genetic elements, which is a major driving force of "S. aureus" evolution. Resistance is conferred by the "mecA" gene, which codes for an altered penicillin-binding protein (PBP2a or PBP2') that has a lower affinity for binding β-lactams (penicillins, cephalosporins, and carbapenems). This allows for resistance to all β-lactam antibiotics, and obviates their clinical use during MRSA infections. Studies have explained that this mobile genetic element has been acquired by different lineages in separate gene transfer events, indicating that there is not a common ancestor of differing MRSA strains. Interestingly, one study suggests that MRSA sacrifices virulence, for example, toxin production and invasiveness, for survival and creation of biofilms