SHMT1 and cancer: Treatment with typically used concentrations of metformin required to induce anti-cancer effects in in vitro studies (5–10 mmol/L)—a range of concentrations that might be achieved in the mitochondrial matrix upon heavy accumulation of metformin due to the mitochondrial membrane potential [16,48,49]—notably reduced the release of M + 0 and M + 1 formate in SHMT1 KO-HAP1 cells (Figure 5A).