Studies in transgenic mouse models [18] and in patients with in situ carcinoma of the breast [19] firmly corroborated the concept that hematogenous dissemination is an early event in breast cancer progression, and unveiled the mechanism at the basis of early diffusion, which seems to involve cell density, progesterone receptor signaling and HER2 signaling, irrespective of the breast cancer subtype [20]. This evidence concerns the gene ERBB2 and breast cancer.