Provides background and analysis of three reports which bring into sharp focus the regulatory mechanisms of the c-Src cytoplasmic tyrosine kinase and the hematopoietic cell kinase (Hck). A description of the crystal structure of human c-Src and the hematopoietic cell kinase (Hck); A paper which demonstrates that the kinase activity of Hck can be stimulated by the binding of specific ligands to the Hck non-catalytic domains.