A mucous cyst is a small, fluid-filled sac that forms on the back of the finger near the base of the fingernail. It is a form of ganglion cyst that erupts from the capsule of the joint at the end of the finger, called the DIP joint. The cyst is attached to the joint capsule by a "stalk" that allows fluid to move into the cyst from the joint. Mucous cysts most commonly affect the index finger of the dominant hand.