The present invention relates to the identification and cloning of a novel gene, MARS (Modulator of Antigen Receptor Signaling), which is a putative tumor suppressor gene. The present invention also relates to the relation of the MARS gene to human cancers and its use in the diagnosis and prognosis of human cancer. The invention also relates to the therapy of human cancers which have a mutation in the MARS gene, or lack the MARS gene, including gene therapy, protein replacement therapy and protein mimetics. Finally, the invention relates to the screening of drugs for cancer therapy.