The crystal structure of the MyD88: IRAK4: IRAK2 death domain (DD) complex reveals a left-handed helical oligomer consisting of 6 MyD88, 4 IRAK4, and 4 IRAK2 DDs. This complex forms a tower-like structure with four layers, where MyD88 is at the bottom, IRAK4 in the middle, and IRAK2 at the top. The helical assembly is hierarchical, with MyD88 recruiting IRAK4 and the MyD88: IRAK4 complex recruiting IRAK2 or IRAK1. This assembly brings the kinase domains of IRAKs into proximity for phosphorylation and activation. The complex has composite binding sites that facilitate the recruitment of individual DDs, confirmed by mutagenesis and known signaling mutations. The specificity of Myddosome formation is determined by molecular complementarity and surface electrostatics. The complex serves as a template for Toll signaling in Drosophila and provides an elegant mechanism for DD complex assembly and regulation in signal transduction. Toll-like receptors (TLRs) and IL-1/IL-18 receptors share a TIR domain and belong to the TLR/IL1-R superfamily. TLRs recognize pathogen-associated molecular patterns (PAMPs) to initiate immune responses. Dysregulation of TLR signaling is linked to various diseases, including inflammatory disorders, autoimmune diseases, and cancer. Signal transduction begins with the approximation of TIR domains upon PAMP or cytokine binding, leading to the recruitment of TIR-containing adaptors like MyD88. MyD88 is critical for signaling responses of IL-1, IL-18, and most TLRs. MyD88 contains an N-terminal DD and a short ID, interacting with IRAKs. IRAK1, IRAK2, and IRAK4 are essential for signaling, while IRAK-M is a negative regulator. Mutations in MyD88 and IRAK4 cause recurrent infections in humans. The structure of the MyD88: IRAK4: IRAK2 DD complex was determined at 3.4 Å resolution using heavy atom derivatives. The complex forms a helical oligomer with a 98° rotation and 6 Å translation between adjacent DDs. The structure shows that MyD88 has a unique H1-H2 loop and distinct features compared to IRAK4 and IRAK2. The complex has three types of interactions: type I, II, and III, with type I and II interactions being more extensive. The complex forms a helical assembly with a 3.7 DD per turn, and the DDs form a staggered hexagonal pattern. The complex has composite binding sites that are critical for IRAK4 and IRAK2 recruitment. The specificity of Myddosome formation is influenced by surface charge and shape complementarityThe crystal structure of the MyD88: IRAK4: IRAK2 death domain (DD) complex reveals a left-handed helical oligomer consisting of 6 MyD88, 4 IRAK4, and 4 IRAK2 DDs. This complex forms a tower-like structure with four layers, where MyD88 is at the bottom, IRAK4 in the middle, and IRAK2 at the top. The helical assembly is hierarchical, with MyD88 recruiting IRAK4 and the MyD88: IRAK4 complex recruiting IRAK2 or IRAK1. This assembly brings the kinase domains of IRAKs into proximity for phosphorylation and activation. The complex has composite binding sites that facilitate the recruitment of individual DDs, confirmed by mutagenesis and known signaling mutations. The specificity of Myddosome formation is determined by molecular complementarity and surface electrostatics. The complex serves as a template for Toll signaling in Drosophila and provides an elegant mechanism for DD complex assembly and regulation in signal transduction. Toll-like receptors (TLRs) and IL-1/IL-18 receptors share a TIR domain and belong to the TLR/IL1-R superfamily. TLRs recognize pathogen-associated molecular patterns (PAMPs) to initiate immune responses. Dysregulation of TLR signaling is linked to various diseases, including inflammatory disorders, autoimmune diseases, and cancer. Signal transduction begins with the approximation of TIR domains upon PAMP or cytokine binding, leading to the recruitment of TIR-containing adaptors like MyD88. MyD88 is critical for signaling responses of IL-1, IL-18, and most TLRs. MyD88 contains an N-terminal DD and a short ID, interacting with IRAKs. IRAK1, IRAK2, and IRAK4 are essential for signaling, while IRAK-M is a negative regulator. Mutations in MyD88 and IRAK4 cause recurrent infections in humans. The structure of the MyD88: IRAK4: IRAK2 DD complex was determined at 3.4 Å resolution using heavy atom derivatives. The complex forms a helical oligomer with a 98° rotation and 6 Å translation between adjacent DDs. The structure shows that MyD88 has a unique H1-H2 loop and distinct features compared to IRAK4 and IRAK2. The complex has three types of interactions: type I, II, and III, with type I and II interactions being more extensive. The complex forms a helical assembly with a 3.7 DD per turn, and the DDs form a staggered hexagonal pattern. The complex has composite binding sites that are critical for IRAK4 and IRAK2 recruitment. The specificity of Myddosome formation is influenced by surface charge and shape complementarity