Four-week-old female BALB/c nude mice were obtained from the Experimental Animal Center of Guangzhou University of Chinese Medicine (Guangzhou, China) and kept in pathogen-free conditions for 1 week. The mice were randomly divided into two groups (n=12 for each group). miR-20a or NC was transfected into MDA-MB-231 cells by lentivirus infection. Stable cells were trypsinized and suspended in PBS. Each mouse was subcutaneously injected with approximately 5 × 106/0.2 ml cells at the abdominal region. The tumor size was measured using digital calipers daily for 3 weeks.