The most commonly known SNPs are in the genes that code for methylation systems.
Methylation is a process that uses a series of enzymes to add a little carbon group, called a methyl group, to other biochemical items. Methylation is used as a locking mechanism for DNA, basically turning on and off genes, which helps appropriate division and formation of immune, gut and hormone producing cells. And because the body is very smart, this one process gets used for other things too like detox pathways, or even making neurotransmitters. MTHFR enzyme is the most important one in this cycle as it recycles the methyl group and funnels more in dietary sources. With enough starting products and vitamin and mineral cofactors, the methyl group can be recycled to be used again and again. In essence, the MTHFR enzyme acts as a central cog fueling several very key and diverse biological functions. The gene that codes for this enzyme is also called the MTHFR gene and it has well-known SNPs, variants that cause that critical enzyme to be a less efficient. We know that an injured or less efficient enzyme is slow and the materials it is supposed to act on build up, as it tries to keep up it burns through its cofactors depleting them.