Apoptosis is an important process by which normal tissue homeostasis and function are maintained. One of the major signals that regulate this process is mediated by the activation of the Fas receptor by its ligand. This leads to the formation of a Fas-associated death domain (FADD)-containing death-inducing signaling complex and the activation of caspase-8, which in turn activates downstream effector caspases, such as caspase-3 and -7 (1). Recent experiments have shown that overexpression of Toso, a novel regulator of Fas-induced apoptosis in lymphoid cells, in Jurkat cells as well as transgenic mice render these cells resistant to Fas-induced apoptosis but not to TRAIL-induced apoptosis (2,3). Furthermore, Toso was found to associate with FADD, suggesting that Toso functions by disrupting the formation of the death-inducing signaling complex. Despite its predicted molecular weight, Toso often migrates at 60kD in SDS-PAGE.