a 72 kDa protein-tyrosine kinase; ligation of the B cell antigen receptor leads to the rapid phosphorylation of the enzyme on tyrosine & to the activation of the tyrosine kinase activity of PTK72 for both substrate phosphorylation & autophosphorylation; also undergoes a second phase of tyrosine phosphorylation & activation as B cells progress through the late G(1) phase of the cell cycle
