 The type of P450 present also determines where the oxygen will be added; phenobarbital (PB)-induced P450s catalyze oxygenation to the meta-para positions of PCBs while 3-methylcholanthrene (3MC)-induced P450s add oxygens to the "ortho"–"meta" positions.PCBs containing "ortho"–"meta" and "meta"–"para" protons can be metabolized by either enzyme, making them the most likely to leave the organism.However, some metabolites of PCBs containing "ortho"–"meta" protons have increased steric hindrance from the oxygen, causing increased stability and an increased chance of accumulation.