PTP-S2⁄TC45 is a nuclear protein tyrosine phosphatase that activates p53 and induces caspase 1-dependent apoptosis. We analyzed the role of ICE protease-activating factor (Ipaf), an activator of caspase 1 in p53-depend-ent apoptosis. We also determined the sequence of events that lead to apoptosis upon caspase 1 activation by Ipaf.