Abstract Glycine cleavage system (GCS) occupies a key position in one-carbon (C1) metabolic pathway and receives great attention for the use of C1 carbons like formate and CO2 via synthetic biology. In this work. we demonstrate that formaldehyde exists as a substantial byproduct of the GCS reaction cycle. Three causes are identified for its formation. https://www.lightemupsequences.com/limited-save-Custom-Congratulations-Banner-p4839-hot-choice/