Export adapter involved in nuclear export of spliced and unspliced mRNA. Binds mRNA which is thought to be transferred to the NXF1-NXT1 heterodimer for export (TAP/NFX1 pathway) (PubMed:15833825, PubMed:15998806, PubMed:17190602, PubMed:11707413, PubMed:11675789, PubMed:11979277, PubMed:18364396, PubMed:22144908, PubMed:22893130, PubMed:23222130, PubMed:25662211). Component of the TREX complex which is thought to couple mRNA transcription, processing and nuclear export, and specifically associates with spliced mRNA and not with unspliced pre-mRNA (PubMed:15833825, PubMed:15998806, PubMed:17190602). TREX is recruited to spliced mRNAs by a transcription-independent mechanism, binds to mRNA upstream of the exon-junction complex (EJC) and is recruited in a splicing- and cap-dependent manner to a region near the 5' end of the mRNA where it functions in mRNA export to the cytoplasm (PubMed:15833825, PubMed:15998806, PubMed:17190602). TREX recruitment occurs via an interaction between ALYREF/THOC4 and the cap-binding protein NCBP1 (PubMed:15833825, PubMed:15998806, PubMed:17190602). The TREX complex is essential for the export of Kaposi's sarcoma-associated herpesvirus (KSHV) intronless mRNAs and infectious virus production; ALYREF/THOC4 mediates the recruitment of the TREX complex to the intronless viral mRNA (PubMed:18974867). Required for TREX complex assembly and for linking DDX39B to the cap-binding complex (CBC) (PubMed:15998806, PubMed:17984224). In conjunction with THOC5 functions in NXF1-NXT1 mediated nuclear export of HSP70 mRNA; both proteins enhance the RNA binding activity of NXF1 and are required for NXF1 localization to the nuclear rim (PubMed:19165146). Involved in the nuclear export of intronless mRNA; proposed to be recruited to intronless mRNA by ATP-bound DDX39B. Involved in transcription elongation and genome stability (PubMed:12438613, PubMed:17984224). Involved in mRNA export of C5-methylcytosine (m5C)-containing mRNAs: specifically recognizes and binds m5C mRNAs and mediates their nucleo-cytoplasmic shuttling (PubMed:28418038). Acts as chaperone and promotes the dimerization of transcription factors containing basic leucine zipper (bZIP) domains and thereby promotes transcriptional activation. Homomultimer. Is part of several complexes involved in mRNA processing and export. Component of the transcription/export (TREX) complex at least composed of ALYREF/THOC4, DDX39B, SARNP/CIP29, CHTOP and the THO subcomplex; TREX seems to have a dynamic structure involving ATP-dependent remodeling; in the complex interacts (via C-terminus) directly with DDX39B and interacts directly with THOC1 and THOC2. Found in mRNA splicing-dependent exon junction complexes (EJC). Identified in the spliceosome C complex. Found in a mRNP complex with UPF3A and UPF3B. Interacts with RBM8A, NCBP1, THOC5, LEF1, RUNX1, EIF4A3, RNPS1, SRRM1, IWS1 and EXOSC1. Interacts with RBM15B (PubMed:19586903). Interacts with NXF1; the interaction is direct. (Microbial infection) Interacts with human Kaposi's sarcoma-associated herpesvirus (HHV-8) ORF57 protein; this interaction allows efficient export of HHV-8 early and late intronless transcripts. (Microbial infection) Interacts with HHV-1 ICP27 protein; this interaction recruits ALYREF to viral replication compartments and probably directs viral mRNA to the TAP/NFX1 pathway. Colocalizes with the core EJC, ALYREF/THOC4, NXF1 and DDX39B in the nucleus and nuclear speckles. Travels to the cytoplasm as part of the exon junction complex (EJC) bound to mRNA (PubMed:19324961). Localizes to regions surrounding nuclear speckles known as perispeckles in which TREX complex assembly seems to occur (PubMed:23826332). Expressed in a wide variety of cancer types. Arg-204 is dimethylated, probably to asymmetric dimethylarginine. Arginine methylation reduces RNA binding. Citrullinated by PADI4. Antibodies against ALYREF/THOC4 are found in sera of patients with systemic lupus erythematosus (SLE). Belongs to the ALYREF family.