  We discuss the heavy-quark contribution to deep inelastic scattering in the
scheme with $n_f=3,4,5$ fixed flavors. Based on the recent ABM11 PDF analysis
of world data for deep-inelastic scattering and fixed-target data for the
Drell-Yan process with the running-mass definition for heavy quarks we show
that fixed flavor number scheme is sufficient for describing the
deep-inelastic-scattering data in the entire kinematic range. We compare with
other PDF sets and comment on the implications for measuring the strong
coupling constant $\alpha_s(M_Z)$.
