Fyn is a membrane-associated, non-receptor protein tyrosine kinase approximately 59kDa, which belongs to the Src family of cytoplasmic tyrosine kinases. Fyn is very strongly similar to mouse Fyn, v-yes and c-src. Fyn is expressed predominately in tissues of neuronal and hematopoietic origin. Neuronal Fyn and hematopoietic Fyn differ at the junction of the SH2 and kinase domains due to tissue specific alternative splicing. Fyn has been shown to be involved in B cell and T cell activation as well as keratinocyte differentiation. In T cells, Fyn associates with the T-cell antigen receptor and Thy1.