STK11 and neoplasm: Here we report that binding of LKB1 to membranes by direct interaction with phospholipids, in particular to phosphatidic acid, is essential for its function in vivo during development of Drosophila. Membrane association of LKB1 is required for its kinase activity and for efficient activation of AMPK in cultured mammalian cells, thus contributing to the tumour suppressor function of LKB1.