Lck (lymphocyte-specific protein tyrosine kinase), with 509-amino acid protein (about 56kDa), belongs to the Src non-receptor tyrosine kinases family. By virtue of common structural motifs, the Src family is composed of nine members in vertebrates, including Src, Yes, Fgr, Frk, Fyn, Lyn, Hck, Lck and Blk. Lck is expressed predominantly in T cells and is localized to the inner surface of the plasma membrane. Lck is activated after T cell stimulation and is required for T-cell proliferation and IL-2 production. Aberrant expression or activation of Lck kinase has been reported in both lymphoid and nonlymphoid malignancies. In addition, inhibition of Lck has been a target to prevent lymphocyte activation and acute rejection.