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The golden Au16- cage is doped systematically with an external atom of different valence electrons: Ag, Zn, and In. The electronic and structural properties of the doped clusters, MAu16- (M =Ag,Zn, In), are investigated by photoelectron spectroscopy and theoretical calculations. It is observed that the characteristic spectral features of 16-, reflecting its near tetrahedral (Td) symmetry, are retained in the photoelectron spectra of MAu16-, suggesting endohedral structures with little distortion from the parent Au16- cage for the doped clusters. Density functional calculations show that the endohedral structures of MAu16- with Td symmetry are low-lying structures, which give simulated photoelectron spectra in good agreement with the experiment. It is found that the dopant atom does not significantly perturb the electronic and atomic structures of Au16-, but simply donate its valence electrons to the parent Au16- cage, resulting in a closed-shell 18-electron system for Ag@ Au16-, a 19-electron system for Zn@ Au16- with a large energy gap, and a 20-electron system for In@ Au16-. The current work shows that the electronic properties of the golden buckyball can be systematically tuned through doping.