The present invention provides methods for determining whether a subject has neoplasia, methods for assessing the efficacy of therapy to treat neoplasia in a subject, methods for assessing the prognosis of a subject who has neoplasia, and methods for treating neoplasia in a subject in need of treatment. The present invention further provides a kit for use in detecting neoplasia. Additionally, the present invention provides a pharmaceutical composition, comprising a modulator of HAUSP expression or a HAUSP protein and a pharmaceutically acceptable carrier. Also provided is a method for deubiquitinating p53 in a cell. The present invention is further directed to a method for identifying an agent that is reactive with p53, and the agent identified by this method. Furthermore, the present invention provides a method for treating a p53-associated condition in a subject in need of treatment. The present invention is also directed to a complex comprising p53 and HAUSP, and a mutant protein comprising the HAUSP amino acid sequence, in which Ser is substituted for Cys at amino acid residue 223. Finally, the present invention provides a transgenic non-human animal whose genome comprises a disruption in its endogenous HAUSP gene.