(2.1.33) Gritsenko : DFT can be formally considered as the result of the simplest exact functional closure of the conventional expression for the nonrelativistic ground-state energy E[ρ, γ, P], which includes the electron density ρ, the first-order reduced density matrix (1RDM) γ, and the diagonal part P of the 2RDM Γ corresponding to a ground-state wave function Ψ. This can be achieved in the spirit of the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) chain of the quantum dynamical reduced theories of many-electron systems. Truncation of BBGKY chain with its exact or approximate closure at the mth level produces theories that operate with the mth (and lower) order RDMs . In this sense, DFT can be considered as the result of the exact closure at the "zero" (i.e., only density functional) level of E[ρ, γ, P] with two maps, in complete analogy with those employed in the derivation of time-dependent density-matrix-functional theory (TDDMFT) . The first map is the evident map P ← Γ → γ → ρ, while the second map ρ → Ψ → Γ employs the Hohenberg-Kohn theorem. It is its simplicity and compactness in the BBGKY sense and also its definite connection with a real world via its exactness that make DFT such a fertile ground for the present wealth of DFAs.