One of the key molecules involved in the control of eIF2a phosphorylation is the protein phosphatase 1 co-factor GADD34, which relieves translation repression during ER stress by promoting eIF2a dephosphorylation [50, 51] , [52] . GADD34 is a direct downstream transcription target of ATF4 [53] . Expression of GADD34 was quantified by qPCR and immunoblot in WT and PKR 2/2 MEFs (Fig. 2B) . In WT cells GADD34 mRNA expression was clearly up-regulated (20 fold) in response to poly I:C, while GADD34 protein induction was equivalent in poly I:Cand thapsigargin-treated cells. GADD34 mRNA transcription and translation were not observed in PKR 2/2 cells responding to poly I:C, but occurred normally upon thapsigargin treatment, paragoning eIF2a phosphorylation (Fig. 2B, right) .