We found that active DNA demethylation activity of DNMT3A is essential for hypoxia-induced EMT of the SW480 colon cancer cells, its global genomic DNA demethylation, and promoter DNA demethylation/transcriptional activation of EMT-associated genes including TWIST1 and SNAIL1. DNMT3A also regulates hypoxia-induced HIF-1α binding to and transcriptional activation of the TWIST1 promoter as well as genome-wide DNA demethylation and EMT of breast cancer and liver cancer cells. The gene discussed is SNAI1; the disease is breast cancer.