Fyn is a membrane-associated tyrosine kinase implicated in the control of cell growth. Fyn protein associates with the p85 subunit of phosphatidylinositol 3-kinase and interacts with the fyn-binding protein. There exist alternatively spliced transcript variants encoding distinct isoforms of Fyn.
The intracellular tails of CD3 molecules contain a single conserved motif termed the immunoreceptor tyrosine-based activation motif (ITAM) that is essential for TCR signaling. Phosphorylation of CD3's ITAM enables the CD3 chain to bind Fyn, making this kinase important in the T cell's signaling cascade.