  High luminosity accelerators have greatly increased the interest in
semi-exclusive and exclusive reactions involving nucleons. The relevant
theoretical information is contained in the nucleon wavefunction and can be
parametrized by moments of the nucleon distribution amplitudes, which in turn
are linked to matrix elements of three-quark operators. These can be calculated
from first principles in lattice QCD. However, on the lattice the problems of
operator mixing under renormalization are rather involved. In a systematic
approach we investigate this issue in depth. Using the spinorial symmetry group
of the hypercubic lattice we derive irreducibly transforming three-quark
operators, which allow us to control the mixing pattern.
