A non-cancerous lump, often on the tendons or joints of wrists and hands. The cause of ganglion cysts is unknown. A ganglion cyst is round, small and usually painless. Although rare, it can be painful if it presses a nerve. In a few cases, the cyst can interfere with joint movement. A ganglion cyst often clears on its own. If troublesome, it can be drained or removed.