The simplest of all nuclei, the deuteron.
The mass of the deuteron is 1875.63 MeV, while the combined mass of a neutron and a proton is 1877.85 MeV. Thus the binding energy of a deuteron is only 2.22 MeV, the difference between [mn + mp]c2 and Mdc2.

Notice that the strong nuclear potential energy between the n and p becomes repulsive at short distances. This is because at short distances, the internal state functions of n and p begin to overlap and this is forbidden by the Pauli Principle, since both n and p are made of quarks, and so both systems contain identical fermions (quarks) in the same state.  This is similar to the situation when two atoms interact to form a molecule... again the potential has a repulsive core, due to the Pauli Principle.