Mammalian Toll-like receptors (TLRs) recognize conserved products of microbial metabolism and activate NF-κB and other signaling pathways through the adapter protein MyD88. Although some cellular responses are completely abolished in MyD88-deficient mice, TLR4, but not TLR9, can activate NF-κB and mitogen-activated protein kinases and induce dendritic cell maturation in the absence of MyD88. These differences suggest that another adapter must exist that can mediate MyD88-independent signaling in response to TLR4 ligation. We have identified and characterized a Toll–interleukin 1 receptor (TIR) domain–containing adapter protein (TIRAP) and have shown that it controls activation of MyD88-independent signaling pathways downstream of TLR4. We have also shown that the double-stranded RNA-binding protein kinase PKR is a component of both the TIRAP- and MyD88-dependent signaling pathways.