LAMB2 and breast cancer: That local invasion occurs first through the thin layer of basement membrane composed primarily of collagen IV and laminins that surrounds tumors and then through the dense extracellular matrix of the breast that is dominated by the presence of fibrillar collagen I. Given that localized breast cancers can only become metastatic if they can breach the basement membrane and invade collagen I-rich environments, either basement membrane or collagen I may be an appropriate in vitro environment in which to assess a breast cancer’s ability to invade.