Core collapse supernovae are dominated by energy transport from neutrinos. Therefore, some supernova properties could depend on symetries and features of the standard model weak interactions. The cross section for neutrino capture is larger than that for antineutrino capture by one term of order the neutrino energy over the nucleon mass. This reduces the ratio of neutrons to protons in the $��$-driven wind above a protoneutron star by approximately 20 % and may significantly hinder r-process nucleosynthesis.

5 pages, deleted "high brow stuff" on charge conjugation for referee, Phys. Rev. Lett. in press