IL1z is a member of the interleukin 1 cytokine family. This cytokine can bind to, and may be a ligand for interleukin 18 receptor (IL18R1/IL-1Rrp). This cytokine also binds to interleukin 18 binding protein (IL18BP), an inhibitory binding protein of interleukin 18 (IL18), and subsequently forms a complex with IL18 receptor beta subunit, and through which it inhibits the activity of IL18. This gene along with eight other interleukin 1 family genes form a cytokine gene cluster on chromosome 2. Five alternatively spliced transcript variants encoding distinct isoforms have been reported.

Isoform A, isoform B and isoform C are expressed in testis, colon, placenta, lung and lymph node. Isoform D and isoform E were found only in testis and bone marrow. Whereas only isoform A is found in brain, only isoform B in kidney and only isoform C in heart.