We assume that strong and electroweak interactions become strong at very high energies. With this assumption, we compute the low-energy gauge coupling constants α_i (m_Z) as a function of the cutoff scale, taking the supersymmetric standard model with 3+2n families of quark and lepton multiplets. We find that only the five family case (n=1) is consistent with the experimental values of the gauge coupling constants. This suggests the presence of a pair of families at ∼ 1 TeV.