EIF2AK4 and hydrops fetalis: Arguing further against the necessity for ribosome collisions in the activation of GCN2 by HF, pretreatment with puromycin, which prematurely released nascent polypeptides and potently lowered ribosome collisions, did not affect HF reductions of tRNAPro charging or HF activation of GCN2 (Figure 2D and E).