tokens: Disruption of constitutive Jak3/Stat5 activation by AG-490 was demonstrated by inhibition of 1 ) tyrosine phosphorylation of Jak3 , Stat5a ( Tyr ( 694 ) ) , and Stat5b ( Tyr ( 699 ) ) ; 2 ) serine phosphorylation of Stat5a ( Ser ( 726 ) ) as determined by a novel phosphospecific Ab ; and 3 ) Stat5a/b DNA binding to the Stat5-responsive beta-casein promoter .