**1. Introduction**

246 Breast Cancer – Focusing Tumor Microenvironment, Stem Cells and Metastasis

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For many reasons, mesenchymal stem cells (MSCs) have lately received much attention. Their plasticity, their tropism for wounds and cancer, their ability to assist in tissue regeneration, their immunomodulary activities, their effects on cancer development and finally their usefulness as drug-delivery vectors made MSCs a prime target for many researchers worldwide. Many aspects of MSC functions have been covered by recent reviews (Beyer Nardi & da Silva Meirelles, 2006; Kidd et al., 2008; Klopp et al., 2011; Krabbe et al., 2005; Patel et al., 2008; Uccelli et al., 2008; Wislet-Gendebien et al., 2005; Yen & Yen, 2008). In this review, we are summarizing the current knowledge on the communication of MSCs with breast cancer cells and its consequences for breast cancer progression.
