Component the THO subcomplex of the TREX complex, which operates in coupling transcription elongation to mRNA export. The THO complex is recruited to transcribed genes and moves along the gene with the elongating polymerase during transcription. THO is important for stabilizing nascent RNA in the RNA polymerase II elongation complex by preventing formation of DNA:RNA hybrids behind the elongating polymerase. It functions in cotranscriptional formation of an export-competent messenger ribonucleoprotein particle (mRNP) by facilitating the loading of ATP-dependent RNA helicase SUB2 and the mRNA export factor YRA1 along the nascent mRNA. Component of the THO complex, which is composed of HPR1, MFT1, THO2 and THP2. Together with SUB2, TEX1 and YRA1, THO forms the transcription/export (TREX) complex. THO associates with DNA and RNA in vitro. Present with 2840 molecules/cell in log phase SD medium.