DOC-2/DAB2 is a member of the disable gene family with tumor-inhibitory activity. Its down-regulation is associated with several neoplasms, and serine phosphorylation of its N terminus modulates DOC-2/DAB2's inhibitory effect on AP-1 transcriptional activity. We describe the cloning of DIP1/2, a novel gene that interacts with the N-terminal domain of DOC-2/DAB2. DIP1/2 is a novel GTPase-activating protein containing a Ras GTPase-activating protein homology domain (N terminus) and two other unique domains (i.e. 10 proline repeats and leucine zipper). Interaction between DOC-2/DAB2 and DIP1/2 is detected in normal tissues such as the brain and prostate. Altered expression of these two proteins is often detected in prostate cancer cells. Indeed, the presence of DIP1/2 effectively blocks mitogen-induced gene expression and inhibits the growth of prostate cancer. Thus, DOC-2/DAB2 and DIP1/2 appear to represent a unique negative regulatory complex that maintains cell homeostasis.