Apoptosis plays a major role in normal organism development, tissue homeostasis, and removal of damaged cells. Disruption of this process has been implicated in a vari ety of diseases such as cancer (reviewed in 1). Aven is a recently discovered protein that blocks apoptosis induced by Apaf-1 and caspase-9 (2). It is thought that Aven functions by binding to Bcl-xL, an antiapoptotic member of the Bcl-2 family, and to Apaf-1, possibly interfering with the ability of Apaf-1 to self-associate, suggesting that Aven impedes Apaf-1-mediated caspase activation. Higher levels of Aven mRNA are seen in patients with acute leukemia than in control patients, suggesting that Aven may be useful as a prognostic indicator in leukemia patients (3). Despite its predicted molecular weight, Aven often migrates at 55kD in SDS-PAGE.