Since several prometastatic genes (Plau, Mmp2, Mmp9, Has2, and Has3) were downregulated in Mbd2-KO PyMT tumors, we next used formalin-fixed lung tissue sections collected at the experimental endpoint and stained them with hematoxylin and eosin (H&E) to assess breast tumor metastasis to the lung. This evidence concerns the gene MMP9 and breast neoplasm.