Provided is the use of glycine cleavage system (GCS) inhibitors, such as cysteamine, a RNAi agent or a relevant antibody, in the manufacture of a medicament for the treatment of serine hydroxymethyltransferase 2 (SHMT2) overexpressing tumours or tumour cells. Further provided is an in vitro method for assessing the sensitivity of tumours or tumour cells to a GCS inhibitor by measuring the level of SHMT2 expression in the tumours or tumour cells, and an in vitro method of screening candidate anti-tumour agents by contacting SHMT2 overexpressing tumours or tumour cells with the agent and determining whether the proliferation of the tumours or tumour cells is inhibited.
