Fig. 2. Response of chicken INK4b and ARF to different signalling pathways. (A) CEFs (32-38 PDs) were infected with retroviruses encoding an activated form of human H-Ras (Ras), human E2F1 or SV40 T-antigen (T-Ag), with corresponding empty vector controls (Vec). Following drug selection, the cells were analysed for expression of the exogenous Ras, E2F1 and T-Ag by immunoblotting (B) Samples of total RNA were analysed for the expression of INK4b and ARF RNA by northern blotting, using probes specific for the 3'-untranslated region of each transcript. GAPDH was used to control for loading. (C) CEFs were infected with MC29 virus or MH2 virus as indicated and total RNA was prepared at 2 days post-infection. Expression of INK4b and ARF was assessed by northern blotting, and virus infection was confirmed using a probe representing the entire ALV genome.