Purpose: Genome-wide DNA methylation analyses have identified hundreds of candidate DNA-hypermethylated genes in cancer. Comprehensive functional analyses provide an understanding of the biologic significance of this vast amount of DNA methylation data that may allow the determination of key epigenetic events associated with tumorigenesis. Experimental Design: To study mechanisms of cysteine dioxygenase type 1 (CDO1) inactivation and its functional significance in breast cancer in a comprehensive manner, we screened for DNA methylation and gene mutations in primary breast cancers and analyzed growth, survival, and reactive oxygen species (ROS) production in breast cancer cells with restored CDO1 function in the context of anthracycline treatment. Results: DNA methylation-associated silencing of CDO1 in breast cancer is frequent (60%), cancer specific, and correlates with disease progression and outcome. CDO1 function can alternatively be silenced by repressive chromatin, and we describe protein-damaging missense mutations in 7% of tumors without DNA methylation. Restoration of CDO1 function in breast cancer cells increases levels of ROS and leads to reduced viability and growth, as well as sensitization to anthracycline treatment. Priming with 5-azacytidine of breast cancer cells with epigenetically silenced CDO1 resulted in restored expression and increased sensitivity to anthracyclines. Conclusion: We report that silencing of CDO1 is a critical epigenetic event that contributes to the survival of oxidative-stressed breast cancer cells through increased detoxification of ROS and thus leads to the resistance to ROS-generating chemotherapeutics including anthracyclines. Our study shows the importance of CDO1 inactivation in breast cancer and its clinical potential as a biomarker and therapeutic target to overcome resistance to anthracyclines.