Background: Mutations (mut) in ESR1 have been reported in ER+ breast cancers (BC) as an acquired resistance mut to aromatase inhibitor (AI) therapy. Acquired ERBB2 mut have also been reported in MBC patients (pts) that cause activated ERBB2 signaling. The emergence of acquired secondary mut presents challenges in effective treatment approaches. Methods: Comprehensive genomic profiling was performed on 83 BC samples with 48 metastatic; 34 primary samples. Targeted next-generation sequencing was performed on 562 cancer associated genes in paired tumor and blood DNA (germline) samples. Results: ESR1-mut were found in 23%(11/48) of ER+ MBC tissues with no mut detected in primary ER+ BCs. Mutations-D538G, Y537S and E380Q in the ligand binding domain of ER were the most common alterations, found in 54.5%, 18% and 18% of ESR1 mut samples, respectively. An ER+ HER2- liver biopsy obtained after 20 mos on AI + everolimus had ESR1-D538G and TSC2 structural event. Protein array showed high expression of androgen receptor (AR) and p-AR and activation of ERBB1/2/3, p-SRC and p-4EBP1 in this sample. Further, a functionally uncharacterized ESR1 mut was found in ER+, HER2+ and a triple negative MBC tissue (the primary BC had been ER+). Mut in PI3K pathway (PIK3CA, ARID1A, TSC1/2, PTEN) were present in 8/11 samples with ESR1 mut. Activating mut in ERBB2 were found in 3/83 samples; all 3 were in ER+ MBC samples with one case harboring mut in both ERBB2 and ESR1 (E380Q – uncertain degree of constitutive activity). Interestingly, a BC sample with ER+ HER2- liver met harbored both ERBB2 (V777L) and ERBB3 (E928G) mut; this pt responded well to trastuzumab/pertuzumab (HP) therapy. A pt with ER+ HER2-ERBB2- L755S mut met to gallbladder found after 7 mos on letrozole/palbociclib therapy responded well to HP and T-DM1 therapy. All 3 ERBB2-mut cancers had a CDH1 frameshift mut suggesting enrichment in pretreated lobular MBCs (Ross J. CCR, 2013). Conclusions: This study shows a 23% and 6% ESR1 and ERBB2 mut rate in MBC samples. No ESR1 and ERBB2 mut were present in primary BC samples. Our findings suggest that pts with lobular MBC should be monitored for acquired ERBB2 mut, and that ERBB2 mut may not arise in BCs which harbor known constitutively active ESR1 mut.