X-linked interleukin-1 receptor accessory protein-like 2 (IL1RAPL2) or Interleukin-1 receptor 9 (IL-1R9) is a member of the interleukin 1 receptor family. This protein is similar to the interleukin 1 accessory proteins. IL-1R9/IL1RAPL2 shows restricted expression in the fetal brain and is highly homologous to IL1RAPL, which is reportedly involved in nonsyndromic X-linked mental retardation. IL-1R9/IL1RAPL2 is highly homologous to IL-1R8. Both forms have no known ligands and receptors are found in the fetal brain. IL-1R9/IL1RAPL2 may function as a negative receptor. Both IL1RAPL1 and IL1RAPL2 have novel C-terminal sequences not present in other related proteins. IL-1R9/IL1RAPL2 may be strong candidates for X-linked non-syndromic mental retardation loci, and that molecules resembling IL-1 and IL-18 play a role in the development or function of the central nervous system.