  I report the recent studies\cite{1,2} on the low--energy manifestations of a
minimal extension of the Standard Model based on the quark-lepton symmetry
$SU(4)_V \otimes SU(2)_L \otimes G_R$ of the Pati-Salam type. Given this
symmetry the third type of mixing in the interactions ot the $SU(4)_V$
leptoquarks with quarks and leptons is shown to be required. An additional
arbitrariness of the mixing parameters could allow, in principle, to decrease
noticeably the lower bound on the vector leptoquark mass originated from the
low-energy rare processes. The decays $\mu \rightarrow e \gamma$, $\mu
\rightarrow e \gamma \gamma$, and $\mu \rightarrow e e \bar e$ via the vector
leptoquark are analysed and the specific hierarchy of the decay probabilities
$\Gamma(\mu \rightarrow e e \bar e) \gg \Gamma(\mu \rightarrow e \gamma \gamma)
\gg \Gamma(\mu \rightarrow e \gamma)$ is shown to exist. The upper limits on
the branching ratios at a level of $10^{-18}$ for the $\mu \rightarrow e \gamma
\gamma$ decay and at a level of $10^{-15}$ for the $\mu \rightarrow e e \bar e$
decay are established.
