(A and B) The ability of TGF-β1 (5 ng/ml) to suppress 4T1 organoid growth was abrogated by elevated miR-181a activity (A), while basal 4T1 organoid growth was significantly suppressed by miR-181a inactivation (B). Data are the mean ± SEM of bioluminescent signals obtained in 3 independent experiments completed in triplicate. *P < 0.05, Student’s t test. Original magnification, ×20. (C) Luciferase-expressing 4T1 cells engineered to stably express an miR-181a antagonist (i.e., miArrest 181a) were injected into the lateral tail veins of BALB/c mice (n = 5), and pulmonary tumor outgrowth was monitored by intravital bioluminescent imaging. Data are the mean ± SEM of pulmonary photon flux readings 24 days after injection, while the inset shows representative bioluminescent signals of pulmonary outgrowth measured on day 24. *P < 0.05. (D) Kaplan–Meier survival curves of BALB/c mice (n = 5) from C.