['Mediator complex 19 (Med19) is a pivotal subunit of the Mediator complex, and its aberrant expression is involved in tumourigenesis. We aimed to explore the mechanism by which Med19 promotes the proliferation of breast cancer.', 'Lentivirus-mediated inhibition of Med19, ectopic expression of Med19 and ectopic expression of core-binding factor subunit alpha 2 to translocation 3 (CBFA2T3) were applied in human breast cancer cell lines. Human breast cancer cell proliferation was determined using CCK8 and colony formation assays after lentivirus infection. The expression of Med19, CBFA2T3 and HEB was measured by real-time reverse transcription polymerase chain reaction and Western blotting. The correlation between Med19 and CBFA2T3 expression in tissue from 25 cases of human breast cancer was analysed.', 'In this study, we demonstrate that cell proliferation and colony formation capacity were significantly inhibited after Med19 inhibition in vitro. The expression of CBFA2T3 was distinctly up-regulated in MDA-MB-231 and MCF-7 human breast cancer cells when Med19 was knocked down; however, the expression of HEB, which is targeted by CBFA2T3, was down-regulated. Meanwhile, ectopic expression of Med19 in BT-549 and Hs578T human breast cancer cells inhibited CBFA2T3 expression but enhanced HEB expression. The proliferation capacity of human breast cancer cells was increased when Med19 was overexpressed, but the effect of Med19 up-regulation could be reversed by CBFA2T3 overexpression. Furthermore, a negative correlation between Med19 and CBFA2T3 expression was demonstrated by Western blotting in human breast cancer tissue.'] Does med19 promote breast cancer cell proliferation by regulating CBFA2T3/HEB expression?