  We consider a detailed account on the construction of the heavy-quark parton
distribution functions for charm and bottom, starting from $n_f=3$ light
flavors in the fixed-flavor number (FFN) scheme and by using the standard
decoupling relations for heavy quarks in QCD. We also account for two-mass
effects. Furthermore, different implementations of the variable-flavor-number
(VFN) scheme in deep-inelastic scattering (DIS) are studied, with the
particular focus on the resummation of large logarithms in $Q^2/m_h^2$, the
ratio the virtuality of the exchanged gauge-boson $Q^2$ to the heavy-quark mass
squared $m_h^2$. A little impact of resummation effects if found in the
kinematic range of the existing data on the DIS charm-quark production so that
they can be described very well within the FFN scheme. Finally, we study the
theoretical uncertainties associated to the VFN scheme, which manifest
predominantly at small $Q^2$.
