Metastatic progression is a complex process involving local invasion, followed by dissemination of malignant cells and finally reestablishment of cancer cells at distant sites.Metastatic relapses remain a major challenge for breast cancer patients.Defining the genetic and epigenetic changes that permit metastasis will facilitate more effective patient management and impact directly on reducing mortality rates.Claudins are a family of tight junction proteins involved in maintaining cell-cell interactions, polarity and the differentiated state of epithelial cells.Disruption of this cell-cell adhesion is of major importance in tumor invasion leading to metastasis.In the involuting mammary gland we have identified a member of this family of tight junction proteins, claudin 1, as one of the most highly up regulated gene during mammary tissue regression, and have shown that its expression is tightly regulated during normal mammary gland development.