  A recent D-brane model designed to accommodate a phenomenologically
acceptable fourth generation of chiral fermions was noted to produce an
unexpected additional unbroken nonanomalous U(1) gauge group at the string
scale. We show that the corresponding charges acting on MSSM fields count
baryon and lepton numbers. If broken spontaneously at lower scales, these
U(1)_B and U(1)_L symmetries provide potential avenues for preserving
baryogenesis while nonetheless explaining the suppression of proton decay
(without the need for R parity), as well as the smallness of right-handed
neutrino Majorana masses compared to the string scale.
