LKB1 loss promotes the initiation of ErbB2-induced mammary tumors. (A) Kaplan-Meier analysis, depicting the percentage of tumor-free animals over time in NIC/LKB1+/+and NIC/LKB1fl/fl cohorts. The T50 values represent the time at which 50% of the mice develop their first palpable mammary tumor. n, number of animals analyzed in each cohort. (B) Number of tumor-bearing mammary glands in each cohort. The average number of involved glands is increased in NIC/LKB1fl/fl (7.5 ±1.1) compared with NIC/LKB1+/+ mice (5.4 ±1.4) (***, P< 0.001). (C) Hematoxylin staining of mammary gland whole mounts dissected from 3-month-old NIC/LKB1+/+ and NIC/LKB1fl/fl mice. Arrows indicate the presence of pre-neoplastic lesions. (D) Mammary tumor growth following mammary fat pad injection of NIC tumor cells harboring shRNAs targeting FireFly luciferase (NIC-FF) and NIC mammary tumors with stable LKB1 knockdown (NIC-LKB1 KD). NIC-FF or NIC-LKB1 KD cells were injected in the mammary fat pads of 8-week old SCID/beige mice (n = 10 mice per group) and tumor growth was monitored by bi-weekly calliper measurements (*, P< 0.05).