hCed-X is an evolutionary conserved protein found in several eukaryotes including human, mouse, C. elegans, Arabidopsis, Drosophila, and rice. In this thesis, we demonstrate that hCed-X localizes to the cytokeratin 8/18 (K8/K18) intermediate filaments and nucleolus in non-apoptotic epithelial cells. During apoptosis induced by various agents, hCed-X relocates to intracellular inclusions where it colocalizes with K8 and caspase cleaved-K18 proteins in a caspase-dependent manner. Furthermore, physical interaction between hCed-X and K8/18 was demonstrated by fluorescence resonance energy transfer technique. To explore the biological function of hCed-X, we employed antisense and RNA interference technologies. We found that specific knockdown of endogenous hCed-X protein protects cell from apoptosis induced by the TNF? indicating that hCed-X functions as a positive regulator in this process of apoptosis. Furthermore, we provide evidence suggesting that hCed-X lies downstream of caspase-8 and upstream of caspase-9 in TNF