Background: Despite the clinical impact of breast cancer (BC) brain metastases (BM), their biological complexity still remains poorly understood. We here evaluate the genomic profile of paired primary and metastatic samples to characterize biological changes acquired by BC during metastatization to the brain. Methods: The expression of 758 BC–related genes was evaluated using the BC360 Panel (nCounter platform) in matched primary BC and their associated BM. Intrinsic molecular subtyping was determined using the PAM50 subtype predictor (Parker et al. JCO 2009). Hormone receptor (HR) and HER2 status were evaluated on the BCBM. A False Discovery Rate (FDR) corrected paired two-class SAM was used to identify changes in single genes expression between paired BC and BMs samples. Results: Twenty-one matched primary BC and BMs samples were analyzed. BC subtype evaluated on the BMs was distributed as follows: 24% (N=5) HR-/HER2-, 24% (N=5) HR+/HER2- and 52% (N=11) HER2+. A considerable shift in PAM50 subtype was observed between primary BC and paired BM. While subtype concordance was high for HER2-enriched (HER2-E) tumors (100%), all Luminal A (LumA) primary BCs switched to either HER2-E (75%) or Luminal B (LumB, 25%) on the BM sample, as did 45% of Basal-like BCs (Basal to HER2-E). A clinical switch from HER2- primary BC to HER2+ BM (defined by IHC and ISH) was observed in only one pair (5%). Consistently, HER2-E (p,0.001) and LumB (p=0.001) PAM50 signatures were significantly more expressed, while the LumA (p,0.001) and Normal-like (p=0.001) signatures were significantly less expressed in BMs as compared to paired primary BCs (FDR-corrected Wilcoxon paired signed rank test). No significant change was observed in the expression of the Basal-like signature (p=0.562). Among the 758 evaluated genes, 60 and 318 genes, respectively, were significantly up- and downregulated in BMs as compared to primary BC (FDR,5%). Upregulated genes were enriched, among others, in genes involved in survival and migration (e.g. FGFR4), cell proliferation (e.g. CCNB1, CDK1) and HER2-amplicon (e.g. ERBB2, GRB7). Downregulated genes were enriched in genes involved in immune response (e.g. CD8A, CCL5, PDCD1LG2, TNF), angiogenesis (e.g. PDGFRB) and response to hormonal stimuli (e.g. ESR1, BCL2). Conclusions: Gene-expression profiling of matched primary BCs and BMs shows recurrent gene expression modifications in metastatic samples which might have potential therapeutic implications (e.g. acquisition of HER2-E subtyping and increase in ERBB2 mRNA levels).