  Recently it was shown that in QCD-like theories with $N_f > N$, where $N_f$
is the number of light flavors and $N$ is the number of colors, there are
correlation functions that vanish in perturbation theory and at short distances
receive dominant, calculable contributions from small instantons. Here we
extend the set of such objects to theories with $N_f = N$, which includes real
QCD, and discuss their application as a calibration of lattice computations at
small quark mass. We revisit the related issue of the $u$ quark mass and its
additive renormalization by small instantons, and discuss an alternative test
of $m_u=0$ on the lattice.
