MyD88 recruits unphosphorylated IRAK1 to the signaling complex. IRAK1 is then rapidly activated and autophosphorylates in a region that is outside the kinase domain (Cao et al. 1996). Several pieces of evidence suggest that IRAK4 triggers IRAK1 activation by phosphorylating its kinase activation loop, leading to IRAK1 autophosphorylation (Suzuki et al. 2002): in vitro kinase assays indicate that IRAK1 can be a direct substrate of IRAK4 (Li et al. 2002); IRAK1 phosphorylation by IRAK4 is independent of and precedes IRAK1 activation and autophosphorylation; IRAK1 autophosphorylation is partially inhibited in cells overexpressing a kinase-inactive IRAK4 protein (Li et al. 2002).