Caspase-1 activation in response to Yersinia infection requires Fas-associated death domain protein (FADD) and RIPK1 (25), and the formation of the FADD/RIPK1/caspase-8–containing complex IIa downstream of TAK1 inactivation (26, 52) suggests that caspase-1 activation initially takes place within this complex (53–55). This evidence concerns the gene RIPK1 and Yersinia infectious disease.