Functional inactivation of the v-ErbA oncoprotein switches AEV-transformed ebls from iron-dependent to iron-independent ferritin expression. AEV-transformed ebls (HD3E22) were cultivated in Epotest medium either under proliferation conditions (IGF-1) or induced to terminal differentiation by supplementation with 2 U/mL human Epo, 1.4 nmol/L insulin, 1 μmol/L each of the steroid antagonists ICI 164384 and ZK 112993, the ErbB inhibitor PD 153035, and the c-Kit antagonist EXBW 50 for up to 96 hours (see Materials and Methods). Twenty-four hours before harvesting for Western blot analysis, ebls were switched to identical media containing either Des (50 μmol/L) or Tf (1 mg/mL). Numbers below each pair of lanes represent the fold induction of iron-dependent ferritin expression.