  We point out that in the Standard Model (SM) there is no explanation why
$|V_{23}|^2 + |V_{13}|^2$ is of order $10^{-3}$. A framework is described for
explaining this small mixing, involving the introduction of vector-like quarks.
A symmetry is introduced so that at a first stage $V_{CKM} =
1\>\!\!\!\mathrm{I}$ and only the third generation of quarks acquires a mass.
It is shown that when interactions of vector-like quarks are taken into account
a realistic quark mass spectrum is generated together with a correct $V_{CKM}$
matrix.
