  The relevance of the single-band two-dimensional Hubbard model to
superconductivity in the doped cuprates has recently been questioned, based on
Density matrix Renormalization Group (DMRG) computations on extended t-J models
that found superconductivity over unrealistically broad doping region upon
electron-doping, yet complete absence of superconductivity for hole-doping. We
report very similar results from DMRG calculations on Cu$_2$O$_3$ two-leg
ladder within the parent three-band correlated-electron Hamiltonian. The strong
asymmetry found in our calculations are in contradiction to the deep and
profound symmetry between electron- and hole-doped cuprate superconductors,
apart from their critical temperatures, that has been found from recent
experiments.
