In this talk I would like to address a few issues regarding the seniority structure of nuclei around 100Sn.In particular, I will show that the seniority symmetry is dynamically conserved in j=9/2 shells irrespective of type ofinteraction. This may be a unique phenomenon for nuclear physics, which is characterized by large spinorbit coupling. The collectivity in Sn isotopes may be enhanced by the mutual effect of seniority and pesudospin symmetry. Moreover,I will show for systems with equal number of protons and neutrons, the seniority coupling is broken and, as a result, a new form of spin-aligned proton neutron pair coupling is favored. It should be mentioned that all these works are done in collaboration with or inspired by experimentalists in Stockholm and other groups.