  A simple model is constructed based on the gauge symmetry $SU(3)_c \times
SU(2)_L \times U(1)_Y \times SU(2)_\ell$, with only the leptons transforming
nontrivially under $SU(2)_\ell$. The extended symmetry is broken down to the
Standard Model gauge group at TeV-scale energies. We show that this model
provides a mechanism for baryogenesis via leptogenesis in which the lepton
number asymmetry is generated by $SU(2)_\ell$ instantons. The theory also
contains a dark matter candidate - the $SU(2)_\ell$ partner of the right-handed
neutrino.
