PTEN and neoplasm: For example, when Pten, a gene involved in tumor growth regulation, is inactivated in the stromal fibroblasts of mice mammary glands, in MMTV-ErbB2/neu mice, the spontaneous tumorigenesis observed in mice expressing wild type levels of stromal Pten was decreased compared to mice lacking stromal Pten, a phenomenon associated with both collagen I deposition and increased macrophage infiltration (27).