MTM1 and X-linked myotubular myopathy: The exocytosis of recycling vesicles that emanate from endosomes marked by PI(3)P requires PI(3)P-to-PI(4)P conversion by the PI 3-phosphatase myotubularin 1 (MTM1) (84), an enzyme implicated in X-linked centronuclear myopathy in humans (see section on disease), and the PI 4-kinase PI4K2A.