Methods of treatment, diagnosis, and determining prognosis of subjects with cancer, generally comprising determining levels of glycine metabolism or a mitochondrial 1-carbon (1-C) pathway enzyme, e.g., SHMT2, MTHFD1L, or MTHFD2, and optionally administering an antifolate or an agent that inhibits a mitochondrial 1-carbon (1-C) pathway enzyme, e.g., SHMT2 or MTHFD2.