In cancer, epigenetic changes such as covalent addition of methyl groups to the genomic DNA itself are more prominent than genetic changes. Cytosine-phosphate-guanosine (CpG) methylation negatively affects gene transcription of an adjacent gene. It is thought that DNA methylation significantly contributes to all hallmarks of cancer. Next to being a potential therapy target, DNA methylation is an emerging field of biomarkers. Technically, DNA provides a stable and robust analyte that is particularly suitable for clinical applications. Moreover, there are numerous potential human DNA sources that could facilitate integration of methylation tests in clinical practice. In breast cancer, DNA methylation has shown promise as a potential biomarker for early detection, therapy monitoring, assessment of prognosis or prediction of therapy response. In particular, paired-like homeodomain transcription factor 2 (PITX2) DNA methylation has been validated using a robust assay for paraffin-embedded tissue for clinically relevant outcome prediction in early breast cancer patients treated by adjuvant tamoxifen therapy.