Isospin-violating mass differences and mixing angles: The role of quark masses

The phenomenology of quantum-chromodynamics-based constituent quark models is employed to discuss isospin-violating mass differences and mixing angles which can arise from an intrinsic mass difference between down and up quarks. Such effects appear not only directly from the quark mass difference $m_d-m_u$ but also indirectly via isospin violations induced by this mass difference in the strong interactions and tend to be much larger than electromagnetic effects.