Gottfried sum rule and ū−d̄ distribution in the nucleon

We discuss various possibilities for explaining recent NMC results, which indicate violation of the Gottfried sum rule, including significant contributions from valence‐quark distributions at very small x and SU(2)‐flavor breaking in the sea. We show that pionic contributions to sea‐quark distributions give qualitative agreement with the NMC data. We discuss other experiments which could give a measurement of u−d.