SignificanceWe report the discovery of complete human interleukin-1 receptor (IL-1R)-associated kinase 1 (IRAK-1) deficiency resulting from a de novo Xq28 microdeletion encompassingMECP2andIRAK1in a boy. Like many boys with MECP2 defects, this patient died very early. IRAK-1 is a component of the Toll-like receptor (TLR)/IL-1R (TIR) signaling pathway. Unlike patients with autosomal-recessive complete deficiency of MyD88 or IRAK-4, two other components of the TIR pathway, this patient presented no invasive bacterial infections. We analyzed the impact of human IRAK-1 deficiency in fibroblasts and leukocytes. The role of IRAK-1 in signaling downstream from IL-1R and TLRs differed according to cell type. These findings reveal similarities and differences in the role of IRAK-1 in the TLR and IL-1R pathways between mice and humans.