ERI1 is an RNA exonuclease that binds to the 3'-end of histone mRNAs and degrades them, which suggests an essential role in histone mRNA decay after DNA replication. It binds with high affinity to the stem-loop structure of replication-dependent histone pre-mRNAs and is required for binding the 5'-ACCCA-3' sequence within that stem-loop. Efficient activity depends on 2' and 3'-hydroxyl groups at the last nucleotide of the histone 3'-end. In vitro, the enzyme shows biphasic kinetics: the last three unpaired nucleotides of the flanking sequence are hydrolysed rapidly, while cleavage through the stem is much slower and takes hours.

Beyond histone transcripts, the ERI1 protein can bind other mRNAs and 5.8S ribosomal RNA, and it is required for 3'-end processing of 5.8S rRNA. In vitro it degrades the 3'-overhangs of short interfering RNAs (siRNAs), pointing to a possible role as a regulator of RNA interference (RNAi). It shows no sequence specificity in vitro and only weak DNA exonuclease activity. ERI1-mediated RNA metabolism is reported to play a key role in chondrogenesis.

The ERI1 gene is assigned by HGNC to the exonucleases group and to the microRNA protein coding host genes group. The protein is annotated in the cytoplasm, nucleus and nucleolus. Earlier names include three prime histone mRNA exonuclease 1, with the aliases 3'HEXO and THEX1. Mouse and rat both carry an orthologous Eri1 gene, so the biology can be studied in common rodent models.