GAD1 and breast cancer: In addition, some metastatic tumor cells in the brain overexpress GAD67, an enzyme that converts glutamate to GABA, sustaining an additional metabolic source to promote cancer proliferation in the brain.279 Therefore, GABA transaminase and other proteins linked to the acquired ability of metastatic cells in the brain to feed on GABA and glutamate, as sources of biosynthetic energy, emerge as potential therapeutic targets to treat metastatic breast cancer.