  We emphasise that even in the extreme chiral limit where only the top and
bottom quarks acquire mass, quark mixing is physically meaningful. This implies
that the natural value of $|V_{13}|^2 + |V_{23}|^2$ is of order one, which is
to be compared to its experimental value of order $10^{-3}$ . We show how this
fine-tuning problem of the Standard Model can be solved through an extension of
the Standard Model where vector-like quarks and a complex singlet are
introduced, together with a flavour symmetry. The mixings of the light quarks
are generated through the mixing of the vector-like quarks with the standard
quarks.
