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 (By similarity). Component of the THO subcomplex of the transcription/export (TREX) complex which seems to have a dynamic structure involving ATP-dependent remodeling. Highly expressed in the developing midbrain and the eyes at 24 hpf. The expression becomes restricted afterwards to the posterior part of the midbrain and the midbrain-hindbrain boundary. Belongs to the WD repeat THOC6 family.