  We review the technique of heavy quark mass expansion of various operators
made of heavy quark fields using a semiclassical approximation. It corresponds
to an operator product expansion in the form of series in the inverse heavy
quark mass. This technique applied recently to the axial current is used to
estimate the charm content of the \eta, \eta' mesons and the intrinsic charm
contribution to the proton spin. The derivation of heavy quark mass expansion
for Qbar \gamma_5 Q is given here in detail and the expansions of the scalar,
vector and tensor current and of a contribution to the energy-momentum tensor
are presented as well. The obtained results are used to estimate the intrinsic
charm contribution to various observables.
