TNFSF9 and brain neoplasm: Localized delivery of poly(beta‐amino ester) nanoparticles encapsulating immunostimulatory genes is utilized to reprogram brain tumor cells into tumor‐associated antigen‐presenting cells (tAPCs) by inducing overexpression of costimulatory 4‐1BBL on the surface of brain tumor cells and IL‐12 secreted into the tumor microenvironment.