Interleukin21β( IL21β) is one of the most pleiotropic cytokines and a central regulator of the immune and inflammatory responses. Interleukin21βwas initially discovered within mice and humans and over the last 10 years has been characterized within a wide variety of animals. The IL21βplays a key role in the inflammatory process , enhancing cell2mediated immunity by inducing the growth and proliferation of lymphocytes , connective tissue cells , and by stimulating immune and inflammatory response effector cells. As an immunoregulatory cytokine , IL21βhas the potential to enhance the immune response induced by a vaccine and/ or to modulate the immune response leading to different effector mechanisms. It is produced by many cell types , including monocytes , macrophages , T and B lymphocytes , fibroblasts , and endothelial and epithelial cells. Expression is induced by a diverse range of stimuli , including mitogens , cytokines , and microbial products. There have been considerable evidences provided by biological cross reaction that fish produce IL21βduring immune responses , and the bioactivity of IL21 in fish has been known for over a decade. But only since 1999 , IL21βgene has been cloned from the rainbow trout. And from then on , IL21β gene has been cloned and expressed in many fish confirming that fish produces IL21β gene during immune responses. In mammals it is produced as an inactive precursor that is processed by interleukin converting enzyme (ICE) to give a biologically active‘mature’peptide. There is no signal peptide in IL21β and its mechanism is unknown. This is the special part of the gene structure of the IL21β. And through the program analyzing , we found this special structure in fish IL21βgene. This paper reviews the functions and structure of gene IL21βand the studies of the gene IL21βin fish recently.