The HeavyMethyl (HM) assay is a real-time PCR assay suitable for the qualitative and quantitative DNA methylation analysis of fresh, frozen, or formalin-fixed paraffin-embedded tissues and remote samples, such as serum, plasma, and urine. The HM uses a methylation-specific oligonucleotide blocker and a methylation-specific probe to achieve methylation-specific amplification and detection. A protocol for a duplex real-time PCR for the analysis of the methylation status of the GSTP1 exon1 in prostate tissue samples is presented. This chapter describes the preparation and analysis of a combined HM GSTP1 Exon1 and GSTP1 reference assay.