  Double parton distribution functions (DPDFs) are used in the QCD description
of double parton scattering. The DPDFs evolve with hard scales through QCD
evolution equations which obey nontrivial momentum and valence quark number sum
rules. We describe an attempt to construct initial conditions for the evolution
equations which exactly fulfil these sum rules and discuss its shortcomings. We
also discuss the factorization of the DPDFs into a product of two single parton
distribution functions at small values of the parton momentum fractions.
