1) Spin and flavor sum rules based on a parton model, 2) SU(2) flavor breaking in antiquark distributions

1) We discuss a sum rule of the tensor structure function $b_1(x)$ for spin-one hadrons along with the Gottfried sum rule. Both sum rules are similar in the sense that they are phenomenological ones based on a naive parton model. As the Gottfried sum rule became useful for studying SU(2)$_{flavor}$ breaking in antiquark distributions, the $b_1$ sum rule could provide important information on sea-quark tensor polarization. 2) Pionic contributions to the antiquark distribution $\bar u-\bar d$ and violation of the Gottfried sum rule are discussed. We find that the contributions account for about half of the discrepancy found by the New Muon Collaboration between their result and the sum rule. In order to distinguish various theoretical explanations, we need independent experimental tests, for example, by using Drell-Yan processes. As an application of mesonic calculations, the difference between strange and antistrange quark distributions in the nucleon is briefly discussed.