We have used Zn and Ga, two unique dopants having definite valence states and filled 3d levels, to probe the YBa2Cu3O7 superconductor. A small doping of Ga induces an orthorhombic to tetragonal structural transition, but the values of Tc are as high as 81 K in the tetragonal phase. YBa2(Cu1−xZnx)3O7 retains the same orthorhombic structure as the parent compound, but with highly depressed Tc. The oxygen vacancy order, or the linear chain structure, is found to be not essential for the high Tc. Our data suggest that Ga and Zn preferentially substitute Cu(1) and Cu(2) sites, which leads to the conclusion that the most important feature of the high Tc oxide superconductors is the Cu—O2 planes.