Peutz-Jegers Syndrome (PJS) is a rare hereditary disease characterized by melanocytic macules of the lips, gastrointestinal hamartomatous polyps and an increased risk for many classes of cancer. Mutations in the gene encoding the serine/threonine kinase LKB1 (also designated STK11) are the cause of PJS. LKB1 activity increases upon the binding of a regulatory complex consisting of the STE20-related adaptor-α (STRAD α) pseudo kinase and the calcium binding protein 39 (MO25, also known as CAB39). STRAD and MO25 determine the subcellular localization of LKB1 by initiating its translocation from the nucleus to the cytoplasm, thus regulating the tumor suppressor activity of LKB1. The LKB1/STRAD/MO25 complex acts as an AMP-activated protein kinase kinase (AMPKK). CAB39L (calcium binding protein 39-like), also known as MO25L (MO25-like) or MO2L, is a 337 amino acid protein that is similar to MO25 and is found in the serum of nearly half of all patients diagnosed with acute monocytic leukemia. This suggests a role for CAB39L in carcinogenesis.