THOC1 (THO complex subunit 1), also known as Tho1, P84, HPR1 or P84N5, is a 657 amino acid nuclear matrix protein and is evolutionarily conserved from yeast to humans. THOC1 contains one death domain and is a component of the heteromultimeric THO/TREX (transcription/export) complex along with THOC2, THOC3, BAT1 and ALY. The THO/TREX complex is recruited to transcribed genes and travels along with RNA polymerase II (Pol II) during elongation, coupling elongating Pol II with RNA splicing and export factors. THOC1 is expressed at high levels in breast cancer cells and at relatively low levels in normal epithelia. A reduction of THOC1 in cancer cell lines results in reduced cell proliferation. This suggests that cancer cells are dependent on the high levels of THOC1 expression and therefore THOC1 may be a good target for cancer therapy.