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The inhibition of host cell translation by poliovirus, also called host cell shutoff, occurs early in the infectious cycle, typically only 1.5 to 2.5 h postinfection in HeLa cells. This chapter explores information regarding cleavage of new initiation factors and then discusses information relevant to whether these factors, particularly eukaryotic translation initiation factor 4GI (eIF4GI), are cleaved in vivo by viral or cellular proteases or both. Researchers have discovered evidence for several eIF4Gase activities that cleave eIF4GI at different sites to produce distinct types of eIF4G cleavage products. In the chapter, researchers define eIF4Gase activities based on the size of cleavage products generated (which results from use of alternate cleavage sites) and the mode of induction of the activity. Researchers have identified three types of eIF4Gase activities (termed eIF4Gase-α, elF4Gase-β, eIF4Gase-γ) that can be generated without poliovirus (PV) infection; two of these appear similar to activities also present in infected cells. The sites, which generate the three major N-terminal cleavage products seen on our immunoblots, are 149 amino acids (aa) upstream of the 2Apro cleavage site and 106 aa upstream of the putative eIF4Gase-β cleavage site. Researchers have screened many S10 extracts with this procedure and found that this specific cleavage activity, which is often weak, appeared in approximately 40% of the extracts tested.