Electron-hole asymmetry and superconductivity
  In a solid, transport of electricity can occur via electrons or via holes. In the normal state no experiment can determine unambiguously whether the elementary mobile carriers have positive or negative charge. This is no longer true in the superconducting state: superconductors know the difference between electrons and holes. This indicates that electron-hole asymmetry plays a fundamental role in superconductivity, as proposed by the theory of hole superconductivity. 
J.E. Hirsch