  We modify a recently proposed heterotic model hep-th/0703210, giving three
net-generations of standard model fermions, to get rid of an additional U(1)
factor in the gauge group. The method employs a stable SU(5) bundle on a
Calabi-Yau threefold admitting a free involution. The bundle has to be built as
a deformation of the direct sum of a stable SU(4) bundle and the trivial line
bundle.
