Fyn is a 59 kD protein tyrosine kinase and a member of the src subfamily. There are two reported isoforms of Fyn; FynT found in T cells, B cells and thymocytes, and FynB found in neuronal cells. Fyn has been shown to control cell growth, interact with the p85 subunit of PI3K, control high affinity IgE receptor-mediated mast cell degranulation, and participate in neuronal cell signaling. Fyn can be activated by T cell receptor engagement or anti-CD3 ligation. Fyn is known to be modified by phosphorylation and palmitoylation and has been shown to interact with Fyn binding protein, p85 PI3K, high affinity IgE receptor, and NCAM140. The FYN-59 monoclonal antibody recognizes human and mouse Fyn and has been shown to be useful for Western blotting, immunoprecipitation, and immunohistochemistry.