AbstractPurpose:Women with hormone receptor–positive early breast cancers have a persistent risk of relapse and biomarkers for late recurrence are needed. We sought to identify tumor genomic aberrations associated with increased late-recurrence risk.Experimental Design:In a secondary analysis of Study of Letrozole Extension trial, a case-cohort–like sampling selected 598 primary breast cancers for targeted next-generation sequencing analysis of gene mutations and copy-number gains (CNGs). Correlations of genomic aberrations with clinicopathologic factors and breast and distant recurrence-free intervals (BCFIs and DRFIs) were analyzed using weighted Cox models.Results:Analysis of mutations and CNGs was successfully performed for 403 and 350 samples, including 148 and 134 patients with breast cancer recurrences (median follow-up time, 5.2 years), respectively. The most frequent alterations were PIK3CA mutations (42%) and CNGs of CCND1 (15%), ERBB2 (10%), FGFR1 (8%), and MYC (8%). PIK3CA mutations and MYC CNGs were associated with lower (P = 0.03) and higher (P = 0.004) tumor grade, respectively; a higher Ki-67 was seen in tumor with CCND1, ERBB2, and MYC CNGs (P = 0.01, P < 0.001, and P = 0.03, respectively). FGFR1 CNG was associated with an increased risk of late events in univariate analyses [17/29 patients; BCFI: HR, 3.2; 95% confidence interval (CI), 1.48–6.92; P = 0.003 and DRFI: HR, 3.5; 95% CI, 1.61–7.75; P = 0.002) and in multivariable models adjusted for clinicopathologic factors.Conclusions:Postmenopausal women with hormone receptor–positive early breast cancer harboring FGFR1 CNG had an increased risk of late recurrence despite extended therapy. FGFR1 CNG may represent a useful prognostic biomarker for late recurrence and a therapeutic target.