Progression of the breast cell epigenome from progenitor to malignancy. During progression from progenitor cell to differentiated epithelium, cells exhibit an increasing level of DNA methylation as differentiation options are restricted. During cancer development, much of this genomic methylation is lost, with the exception of a small subset of genomic loci that exhibit DNA hypermethylation. Genomic hypomethylation is associated with increased genomic instability. The methylome of malignant lesions is remarkably similar between early lesions like ductal carcinoma in situ and later lesions like invasive ductal carcinoma.