Minocycline is also known to indirectly inhibit inducible nitric oxide synthase. Proline hydroxylation is also a vital component of hypoxia response via hypoxia inducible factors. Asparagine can be hydroxylated in the HIF1 hypoxia inducible transcription factor. Oxygenases consist of both constitutive and inducible isozymes (HO-1, HO-2). The inducible isoform iNOS produces large amounts of NO as a defense mechanism. In all three classes, constitutive and inducible forms exist. The lactose operon is one example of an inducible system. A positive result is evident from an inducible myocardial perfusion defect. In general, treatment with cephalosporins results in inducible beta-lactamase activity. Other advantages of using RNAi include the availability of inducible and tissue specific knockout. If the lac operator were not present the IPTG would not have an inducible effect. This suggests evolutionary and inducible adaptations to allow consumption of certain host plants. The system is inducible by external signals. The GAL4/UAS system is an example of both an inducible and repressible system. This is an example of the derepressible (from above&colon; negative inducible) model.