Slp4-a/granuphilin-a is a member of the synaptotagmin-like protein (Slp) family and consists of an N-terminal Slp homology domain (SHD) and C-terminal tandem C2 domains. Slp4-a is specifically localized on secretory granules in some endocrine and exocrine cells through its SHD, and it attenuates Ca 2+-dependent dense-core vesicle (DCV) exocytosis when transiently expressed in endocrine cells. Although the SHD of Slp4-a interacts with three distinct Rab species (Rab3A, Rab8A, and Rab27A) in vitro, in contrast to other Slp members, which only recognize Rab27 isoforms, Slp4-a functions as a Rab27A effector during DCV exocytosis under physiological conditions. This chapter describes various approaches that have been used to characterize the function of Slp4-a as a Rab27A effector, rather than a Rab3A or Rab8A effector, both in in vitro and in neuroendocrine PC12 cells. Specifically, the methods that have been used to analyze (1) the physical interaction between Slp4-a and Rab27A, including pull-down assay and cotransfection assay in COS-7 cells; (2) the localization of Slp4-a-Rab27A complex on DCVs in PC12 cells; and (3) the involvement of Slp4-a and Rab27A in DCV exocytosis by neuropeptide Y (NPY) cotransfection assay combined with site-directed mutagenesis are described.