Further steps in the transduction pathway are still unknown. Experimentally, resistance usually arises as a result of mutations in the sterol biosynthesis pathway. Caspases are part of the apoptosis pathway. Glycolysis is an oxygen-independent metabolic pathway. Specifically, it causes a blockage of the nicotinergic neuronal pathway. F-box proteins target other proteins for degradation via the ubiquitin degradation pathway. A microbial biosynthetic pathway has been proposed. Deficiency in the enzymes of the porphyrin pathway leads to insufficient production of heme. The precise molecular mechanisms of the BIR pathway remain unclear. P450 monooxygenases are involved in the amygdalin biosynthetic pathway. Classically, metabolism is studied by a reductionist approach that focuses on a single metabolic pathway. They may be specific to a particular organism, pathway or molecule of interest. Research has been able to show the function of niacin in the pathway lipid metabolism. It is an enzyme in the rutin catabolic pathway. The main degradation pathway of moclobemide is oxidation.