According to my research on studies conducted by various medical professionals, I can say that based on the information provided within the question It is likely that Mitchell carries a gene that boosts the availability of endorphins. These are peptides whose main role is to activate the body's opiate receptor, which cause an effect that relieves pain. Which a gene that boosts this would allow an individual to barely feel pain in situations that would otherwise be very painful such as fracturing a bone.
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