SHMT1 and cancer: The SGOC-related anti-cancer actions of metformin, which should not exclusively be viewed as only a downstream consequence of mCI targeting, but also as a causal upstream inhibitory effect directly involving SHMT2-driven mitochondrial serine catabolism, might illuminate an unforeseen capacity to synergize or antagonize with other interventions involving SHMT1 and/or SHMT2 functioning [10,53,58,59,81,82,83].