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://parisnaturalfoodes.shop/product-category/mag-malate-renew/