a, miR-335, miR-126, miR-122a and miR-199a expression was assessed in a set of 20 primary breast tumour samples through qRT-PCR. Kaplan-Meier curves depict metastasis-free survival of patients whose primary tumours contained low or high levels of the indicated miRNAs. P-values were obtained using a log-rank test. b, The parental MDA-MB-231 cells were transfected with antagomirs targeting endogenous miR-335, miR-199a* or a control antagomir. Four days after transfection, 1 × 104 LM2 cells from each cohort were inoculated intravenously into immunodeficient mice. Lung colonization was measured by bioluminescence at day 35 and quantified. n = 5; error bars represent s.e.m.; P-values based on a one-sided rank-sum test.