We probe the unification of down quarks and leptons in a supersymmetric $\text{SO}(10)$ GUT. The large atmospheric neutrino mixing angle induces $b_R-s_R$ transitions, which can account for the sizeable $CP$ phase $\phi_s$ measured in $B_s-\ovl{B}_s$ mixing. Corrections to down-quark-lepton unification from higher-dimensional Yukawa terms translate neutrino mixing also into $s_R-d_R$ and $b_R-d_R$ currents. We find the flavour structure of Yukawa corrections to be strongly constrained by $\epsilon_K$.