HEL cells were cultured in the presence of medium alone (negative control), TPO (100 ng/mL), Epo (2 U/mL), hemin (10⁻⁷ mol/L), forskolin (10⁻⁵ mol/L), or PMA (10⁻⁷ mol/L) for 30 minutes. CRE-CAT promoter activity was measured as folds of activations with respect to HEL cells left untreated. Data are reported as the means ± SD of four independent transfection experiments performed in duplicate.