- Tyrosine phosphorylation of Blk and Fyn Src homology 2 domain-binding proteins occurs in response to antigen-receptor ligation in B cells and constitutively in pre-B cells. Aoki, Y., Isselbacher, K.J., Cherayil, B.J., Pillai, S. Proc. Natl. Acad. Sci. U.S.A. (1994)