PCBs undergo xenobiotic biotransformation, a mechanism used to make lipophilic toxins more polar and more easily excreted from the body. The biotransformation is dependent on the number of chlorine atoms present, along with their position on the rings. Phase I reactions occur by adding an oxygen to either of the benzene rings by Cytochrome P450. 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.