Background: In depth sequencing of primary breast cancer (BC) has identified a heterogeneous repertoire of disease drivers, evidence of clonal evolution and underlying mutational processes. As cancer evolves over time and under treatment pressure, whole genome sequencing (WGS) of metastatic BC (MBC) tissue is crucial to gain further insight into its genetic make-up and driving forces, thereby allowing improved patient management. Methods: Metastatic tissue and matched germline DNA of patients with MBC (N=501) were prospectively recruited under the biopsy protocol of the Center of Personalized Cancer Treatment (CPCT-02; NCT01855477) and analyzed by WGS. Sequence reads were mapped to the reference genome to call somatic single nucleotide variants (SNV), small insertions and deletions (InDels) and copy number variations from which mutational signatures and tumor mutational burden (TMB; the number of SNV and InDels relative to the genome) were derived. The incidence of called aberrations in our cohort was compared to previously reported WGS data of 560 primary BC (BASIS cohort, Nik-Zainal et al. Nature 2016) (Table 1). Results: According to routine diagnostics 303 patients (60.5%) were ER+/HER2-, 70 (14%) triple negative (TNBC), 95 (19%) HER2+ and the remaining 33 (6.6%) had as of yet an unknown subtype. Top 5 recurrently affected genes were TP53 (47%), ATM (33%), MAP2K4 (32%), NCOR1 (31%), ERBB2 (30%). In the metastatic lesions, median TMB was 2.9 per million base pairs (IQR: 1.7-5.3). Interestingly, 53 (11%) patients had a high TMB (≥10). Compared to primary BC (BASIS cohort), we found (subtype-specific) enrichment of alterations in multiple genes such as ATM (0.4% to 33%) , GPS2 (1.3% to 29%) , MAP2K4 (6.4% to 32%), CBFB (2.7% to 25%) , and, as previously reported, ESR1 (1.3% to 20%). APOBEC signature mutations appeared to be enriched in MBC while HRD signature mutations seemed less prevalent. Analyses to reveal additional genomic features is ongoing as well as the association of genomic alterations with uniquely collected information on prior treatments and response to treatment received directly after biopsy (i.e. endocrine therapy alone or combined with CDK4/6 inhibitors and chemotherapy). Also, to exclude methodological bias the raw data of the BASIS cohort will be processed through our pipeline. Conclusion: WGS of this unique cohort of patients with MBC shows a genetic make-up roughly similar to primary BC, but does show subtype-specific enrichment of selected driver mutations in metastatic disease. This study provides better insight into the tumor biology of MBC potentially improving management of these patients. Citation Format: Angus L, Wilting SM, van Riet J, Smid M, Steenbruggen TG, Tjan-Heijnen VC, Labots M, van Riel JM, Bloemendal HJ, Steeghs N, van de Werken HJ, Lolkema MP, Voest EE, Jager A, Cuppen E, Sleijfer S, Martens JW. The genomic landscape of 501 metastatic breast cancer patients [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr GS1-07.