AbstractLong-noncoding RNAs (lncRNAs) have been shown to participate in oncogenesis across a variety of cancers and may represent novel therapeutic targets. However, little is known about the role of lncRNAs in basal-like breast cancer (BLBC), the aggressive form of breast cancer with no molecularly defined therapeutic target. To examine whether altered lncRNA expression contributes to the aggressive phenotype characteristic of BLBC, we performed a comparative analysis of BLBC versus non-BLBC using microarray profiling and RNA sequencing of primary breast cancer. We identified RP11-19E11.1 as a significantly up-regulated lncRNA in BLBC tumors and named it Basal-Like breast cancer Associated Transcript 1 (BLAT1). Analysis of pan-cancer datasets showed the highest expression of BLAT1 in BLBC tumors compared to all other cancers. Depletion of BLAT1 in breast cancer cells led to significantly increased apoptosis, partly because of accumulation of DNA damage. Mechanistically, BLAT1 expression is regulated at the epigenetic level via DNA methylation at CpG islands in the promoter. Concordantly, patients harboring tumors with BLAT1 hypomethylation showed decreased overall survival. Our results suggest that increased expression of BLAT1 via CpG site hypomethylation may contribute to the aggressive phenotype of BLBC, raising a possibility of new biomarkers for prognosis of aggressive BLBC tumors.