Deciphering genetic determinants of tumorigenesis is the greatest challenge and promise of the present-day era of biomedical research. As extensive tumor genome characterization efforts of the past decade had revealed, tumor genomes harbor multiple point mutations and gene copy number alterations. This exquisite complexity brings forth the challenge of distinguishing numerous incidental alterations from those that are functionally relevant to tumorigenesis. During the past decade, functional genetic screens have shown their utility in identifying genetic changes that functionally contribute to tumor-specific hallmarks and thus hold a great potential for identifying promising new targets for the rational design of successful anticancer therapies. A key hallmark of cancer cells is their ability to escape signals that govern homeostasis of normal tissue. In normal epithelia, growth and survival of cells is dictated by their physical anchorage to the extracellular matrix, and disruption of proper cell-matrix anchorage triggers cell death. Tumors of epithelial origin develop ways to subvert anoikis signals, which enables both their uncontrollable expansion at the primary site as well as metastatic colonization of distant organs. Understanding the genetic determinants of matrix-independent growth of cancer cells is a promising approach to identify potent and selective anticancer targets. In the work presented in this dissertation, we use an unbiased functional genetic screening approach to test a large set of eight thousand human genes to identify those that are involved in inducing and maintaining resistance of mammary epithelial cells to matrix detachment-induced cell death. We show that a cell adhesion molecule PVRL4 promotes cell survival in the absence of matrix anchorage in normal epithelial cells and in cancer cells. Our work reveals that PVRL4 promotes anchorage-independent growth by promoting cell-to-cell attachment and matrix-independent c-Src activation. PVRL4 is focally and frequently amplified in several types of solid tumors. Growth of orthotopically implanted tumors in vivo is inhibited by blocking PVRL4-driven cell-to-cell attachment with monoclonal antibodies, demonstrating a novel strategy for targeted therapy of cancer.