IL-1 (Interleukin-1) is a cytokine responsible for initiating a variety of activities through the activation of transcription factors, NFkB and AP-1, thereby promoting host response to injury or infection. The IL-1 superfamily is comprised of several ligands and receptors. IL-1F7, also known as IL-1?, IL-1 homolog 4 (IL-1H4) or IL-1 related protein 1 (IL-1rp1), is a secreted ligand belonging to this superfamily. It is expressed in the cytoplasm of peripheral monocytic cells localizing to the inner surface of the plasma membrane and surrounding the nuclear membrane suggesting that IL-1F7 may play a role in immune regulation. Five isoforms exist for this protein due to alternative splicing and they are designated IL-1F7a through IL-1F7e. Isoform a has a distinct N-terminus while isoforms c-d are truncated. Isoform b is the mature functional isoform that binds to the IL-18Ralpha-chain and the IL-18 binding protein (IL-18BP) acting as an IL-18 antagonist. IL-1F7 is closely related to IL-1ra, sharing 36% sequence identity.