The present invention relates to compositions and methods for characterizing, diagnosing, and treating cancer. In particular the invention provides the means and methods for the diagnosis, characterization, prognosis and treatment of cancer and specifically targeting cancer stem cells. The present invention provides a soluble FZD receptor comprising an extracellular domain of a human FZD receptor that inhibits growth of tumor cells. The present invention still further provides a soluble receptor comprising a Fri domain of a human FZD receptor that binds a ligand of a human FZD receptor and said soluble receptor is capable of inhibiting tumor growth. The present invention still further provides a method of treating cancer comprising administering a soluble FZD receptor comprising for example, either an extracellular domain of a human FZD receptor or a Fri domain of a human FZD receptor, in an amount effective to inhibit tumor growth.