MicroRNA-125b is a poor prognostic biomarker and represents a therapeutic target to overcome letrozole resistance. (A) Kaplan-Meier analysis (log-rank test) for relapse-free survival (RFS) is shown in a cohort of primary breast tumor samples (n = 65). (B) Effect of the microRNA (miR) miR-125b-5p expression level and lymph node status signature on RFS among our cohort with 65 breast tumors. (C) Cell viability in response to letrozole in Res-Let cells transiently transfected with either miR-125b-5p inhibitor or its negative control. (D) Cell viability in response to letrozole (10−5 M) and/or MK-2206 (10−9 M) in MCF-7aro cells transiently transfected with either miR-125b-5p mimic or its negative control. (C) and (D) are representative of at least three independent experiments. ***P < 0.001 (Student’s t-test).