STK11 and breast cancer: Since mammary tumors from STK11−/−/NIC mice are positive for elevated ErbB2 expression (Fig. 2B), and 31% of HER2 positive breast cancers show loss of LKB1 expression (Table 1, Fig. 1), we investigated the effect of a next generation small molecule that targets the EGFR (ErbB1) and HER2, BIBW2992 (BIBW), on mTOR signaling.