The genomic data suggest that breast cancers arising in the highest and lowest quintiles of MD are very similar with respect to copy number profiles and overall frequency and spectrum of driver gene mutations but with the notable exception of TP53. The low frequency of TP53 mutations observed in high-MD cancers may reflect the cancer growth promoting influence of the rich stromal microenvironment of dense breasts precluding the need for mutation of this strong somatic driver gene. This evidence concerns the gene TP53 and breast cancer.