In a secretory pathway organelle like the Golgi complex, resident proteins are retained in the face of substantial protein flux to subsequent destinations. Recently, molecular genetic strategies have been used to study membrane protein retention in a compartment of the yeast Saccharomyces cerevisiae that is analogous to the trans Golgi network (TGN) of mammalian cells. These studies have defined retention signals containing aromatic amino acids in the TGN proteins' cytoplasmic domains. The identification of mutants that fail to retain TGN proteins has offered the first glimpse into the components involved in retention. The phenotypes of these mutants suggest that retention involves retrieval of TGN proteins from an endosomal compartment.