Sustained activation of STAT5 via the truncated chimeric receptor form EE372. In SKT6-EE372 cells, the time course of STAT5 activation as induced by EGF versus Epo was assayed as follows. Cells were exposed to either EGF or Epo at concentrations shown to promote mitogenesis of myeloid FDCW2-wtER and FDCW2-EE483 cells at 50% maximal rates (35 ng/mL or 90 nmol/L and 20 U/mL or 120 nmol/L, respectively; R.C.G., unpublished data). At the indicated intervals of cytokine exposure, SKT6 cell lysates were prepared by sonication in the presence of CHAPS. Activated STAT5 then was bound to a biotinylated PRE cassette, adsorbed to streptavidin agarose, eluted from washed gels, and assayed by Western blotting. Data shown are representative of two independent experiments.