Provided herein are methods of treating a cancer in a subject comprising administering a FOXP3 protein, a nucleic acid encoding a FOXP3 protein, or an inducing compound which induces FOXP3 protein expression. Methods of altering a phenotype of a cancer cell or tumor cell, methods of inhibiting growth of such cells, and methods of inducing apoptosis of these cells are also provided herein. These methods comprise contacting the cell with a FOXP3 protein, a nucleic acid encoding a FOXP3 protein, or an inducing compound which induces FOXP3 protein expression. Further provided herein are diagnostic methods, comprising comparing the expression or structure of a FOXP3 protein or FOXP3 gene in a test sample to that of a normal or prior sample. A method of screening a test compound for anti-cancer activity comprising administering to cells the test compound and measuring FOXP3 protein or FOXP3 gene expression is moreover provided herein.