DIS heavy-flavor contributions at two loops in a general mass scheme

We discuss an NNLO realization of the general mass scheme S-ACOT-χ for treatment of heavy-flavor production in neutral current deep-inelastic scattering. The connection of this scheme to the QCD factorization theorem with massive quarks is elucidated. As a new feature, kinematical constraints on collinear production of heavy quarks that are crucial near the heavy-quark threshold are included in the amended QCD factorization theorem. Practical implementation of the NNLO calculation is illustrated on the example of semi-inclusive structure functions F2c(x,Q) and FLc(x,Q).