Flavor Dependence of Sea Quark Structure Functions

Quark masses are shown to play an important role in the nucleon sea. Our analysis using massive QCD splitting functions demonstrates the existence of two Feynman-$x$ sea regimes. For small $x$ the strange sea is larger while at large $x$ the non-strange light sea is dominant. This crossover effect has been observed in a phenomenological analysis by the CTEQ Collaboration. We also investigate the charm component in the nucleon.