Lets start with the core idea. A gene is an instruction for building a protein . DNA → RNA → Protein → Biological Function For MTHFR: Gene: MTHFR Protein produced: MTHFR enzyme Job: Convert folate → methylfolate That’s it. But that step is critical  because methylfolate fuels the methylation cycle , which controls: neurotransmitters detox pathways DNA repair hormone metabolism homocysteine levels So a tiny gene variation can ripple through dozens of systems . What a Mutation