  We have found a nilpotent quantum symmetry of Yang-Mills theory restricted to
the Gribov region. In fact, we give a set of transformations for the filds a la
BRST that leaves the Gribov-Zwanziger action invariant and obeys the
nilpotency, although the usual BRST symmetry is broken due to the non-vanishing
Gribov parameter representing the presence of the Gribov horizon in the
configuration space of the Yang-Mills field. We show that the horizon condition
(the gap equation determining the Gribov parameter) follows from the modified
BRST invariance of the vacuum.
