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Hadronic molecules with hidden charm and bottom

Many of the new structures observed since 2003 in experiments in the heavy quarkonium mass region, such as the X (3872) and Z c (3900), are rather close to certain thresholds, and thus can be good candidates of hadronic molecules, which are loose bound systems of hadrons. We will discuss the consequences of heavy quark symmetry for hadronic molecules with heavy quarks. We will also emphasize that the hadronic molecular component of a given structure can be directly probed in long-distance processes, while the short-distance processes are not sensitive to it.