XBP1 and hydrops fetalis: To do so, cell lines were grown in presence of halofuginone hydrobromide (HF), a GCN2 activator already described to be effective in RTH-149 cells,26 in order to confirm that HF treatments were efficient to induce the expression of ISR target genes from the GCN2 (ddit3, asns, and xbp1) and the PERK-dependent unfolded protein response (UPR) pathways (edem1 and grp78 genes) (Figure S3A).