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ERI1, also named as HEXO, 3'HEXO and THEX1, contains one exonuclease domain and one SAP domain. The SAP domain is necessary for binding to the stem-loop structure of histone mRNAs and to form the ternary complex with SLBP, but not for 3'-end histone mRNA exonuclease activity. It is a RNA exonuclease that binds to the 3'-end of histone mRNAs and degrades them, suggesting that it plays an essential role in histone mRNA decay after replication. It is able to bind other mRNAs. ERI1 is required for 5.8S rRNA 3'-end processing. It also binds to 5.8s ribosomal RNA. ERI1 binds with high affinity to the stem-loop structure of replication-dependent histone pre-mRNAs.