AmpC genes can be both, chromosomally encoded or located on a plasmid. Some bacteria within Enterobacteriaceae are almost always carrying an AmpC coding gene on their 'chromosome' (Enterobacter cloacae as a prominent example or the nonfermenter Pseudomonas aeruginosa). This can be repressed or derepressed, thereby having effect on the phenotypic resistance. In addition, Gram negative bacteria may acquire plasmids positive for AmpC coding genes as well. Phenotypically, however, there is no way to discriminate reliably between plasmid or chromosome located ampc.