Background: BC is the most frequent malignancy in women and is potentially sensitive to hormonal treatment (HR+, HER2+ or -) in approximately 70% of cases. PIK3CA is among the most frequently mutated genes in BC (approximately 40% of cases) and its mutations portend sensitivity to PI3K inhibitors, such as alpelisib, in combination with hormonal treatment in HR+/HER2- cases. However, extended NGS-based testing is not currently recommended by national and international guidelines. Methods: Women aged 18 years or older, with cytological or histological diagnosis of HR+/HER2- mBC, for whom suitable tissue samples were available, were subjected to extended NGS profiling using the FoundationOne, analyzing 324 genes and gene signatures, such as tumor mutational burden (TMB) and microsatellite instability (MSI); and Illumina TruSight Oncology 500, analyzing 500 genes. DNA was isolated from formalin-fixed, paraffin-embedded (FFPE) samples, from either the primary or metastatic site. Results: Patients (pts) were analyzed (ductal histology: 52%; lobular: 15%; mixed ductal-lobular: 12%; NOS: 21%), among which 7 (12%) had inadequate pathological material and were considered screening failures. A full molecular report was obtained in 53 patients. The most frequent pathogenic and likely pathogenic alterations (class 5 and 4), listed in descending order of prevalence, are: PIK3CA in 51% of pts (E545K>H1047R>N345K>E542K); CDH1 in 21% (prevalence in lobular carcinoma: 75%; in ductal carcinoma: 3,5%). CCND1 in 28% (all amplifications). FGFR1-2 in 21%, TP53 in 15%, WHSC1L1 and ZNF703 in 13%, RAD21 and TBX3 in 9%. Homologous recombination-related genes, such as BRCA1/2 and PALB2 were altered in <5% while ATM altered in 9% of cases. Many other pathogenic mutations (affecting 69 individual genes) were identified, all with a prevalence <10%. TMB was high (>17 mutations/Mb) in 13% of cases, in one of which MSI was also detected. Conclusions: In our analysis PIK3CA was confirmed to be the most frequently mutated gene (51% of cases); however, due to the specific European regulatory constraints, it was deemed to be actionable in a minority of cases, suggesting that NGS profiling should be performed early on after the diagnosis of metastatic disease, in order to inform the overall therapeutic strategy. Other potentially actionable alterations (FGFR1/2, AKT, HER2 ex 20 mutations) emerged from extended NGS profiling, suggesting its potential utility in selected HR+/HER2- mBC pts.