Required for efficient export of polyadenylated RNA. Acts as component of the THO subcomplex of the TREX complex which is thought to couple mRNA transcription, processing and nuclear export, and which specifically associates with spliced mRNA and not with unspliced pre-mRNA. 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 via the TAP/NFX1 pathway. Regulates transcriptional elongation of a subset of genes. Involved in genome stability by preventing co-transcriptional R-loop formation (By similarity). Participates in an apoptotic pathway which is characterized by activation of caspase-6, increases in the expression of BAK1 and BCL2L1 and activation of NF-kappa-B. This pathway does not require p53/TP53, nor does the presence of p53/TP53 affect the efficiency of cell killing. Activates a G2/M cell cycle checkpoint prior to the onset of apoptosis. Apoptosis is inhibited by association with RB1 (By similarity). Essential for early embryonic development. Required for normal gene expression during postnatal testis development. Component of the THO complex, which is composed of THOC1, THOC2, THOC3, THOC5, THOC6 and THOC7; together with at least ALYREF/THOC4, DDX39B, SARNP/CIP29 and CHTOP, THO forms the transcription/export (TREX) complex which seems to have a dynamic structure involving ATP-dependent remodeling. Binds to the hypophosphorylated form of RB1. Interacts with THOC2, DDX39B and RNA polymerase II (By similarity). Interacts with THOC5 (PubMed:16909111). Interacts with LUZP4 (By similarity). Can shuttle between the nucleus and cytoplasm. Nuclear localization is required for induction of apoptotic cell death. Translocates to the cytoplasm during the early phase of apoptosis execution (By similarity). Widely expressed during embryonic development. An intact death domain is needed for apoptosis. Expression is altered specifically during apoptosis and is accompanied by the appearance of novel forms with smaller apparent molecular mass. Polyubiquitinated, leading to proteasomal degradation; probably involves NEDD4. Mice show early embryonic lethality and severely diminished fertility.