PDZ (an acronym representing three proteins--postsynaptic density protein PSD95/SAP90, drosophila tumor suppressor DLGA, and tight junction protein ZO-1) domain containing proteins are adapter proteins that play indispensable roles in regulating cell growth, development, and differentiation, predominantly through their capacity to serve as central organizers of protein complexes at the plasma membrane. A recently identified member of this protein family is melanoma differentiation associated gene-9 (mda-9), also known as syntenin, which was first identified as a gene down-regulated during human melanoma differentiation as mda-9 and subsequently recognized as an interacting partner of the cell-surface heparan sulfate syndecans, syntenin. Interest in mda-9/syntenin is intensifying because of its involvement in organization of protein complexes in the plasma membranes, regulation of B cell development, intracellular trafficking and cell surface targeting, cancer metastasis, synaptic transmission, and axonal outgrowth. In this review, we discuss the identification, structure and function of mda-9/syntenin and delineate future studies to address its role in regulating key physiological and pathological processes.