The protein REPS2 is implicated in growth factor receptor-mediated endocytosis and signalling, and its expression is downregulated in androgen-independent prostate cancer cells. Herein, the NF-?B subunit p65 is identified as a human REPS2 protein partner, interacting with the EH domain of REPS2. Using crystal structure data from literature and experimental data from yeast and mammalian two-hybrid analysis, the results indicate that the NPF-motif in p65 acts as binding site for the EH domain in REPS2. However, in cultured prostate cancer cells, the REPS2-p65 interaction is triggered upon stimulation with phorbol ester (PMA). This indicates that PMA-sensitive signalling pathways can affect the interaction between REPS2 and p65. During prostate cancer progression from androgen-dependent to androgen-independent growth, downregulation of REPS2 is accompanied by upregulation of NF-?B activity. This might involve loss of REPS2-p65 interaction, which would lead to increased NF-?B activity. Androgen-deprivation causes apoptosis of prostate cancer cells, and activated NF-?B is a known inhibitor of apoptosis. Hence, decreased expression of REPS2 might be a key factor, causing prostate cancer cells to become resistant to induction of apoptosis by androgen deprivation.Oncogene (2004) 23, 5607-5615. doi:10.1038/sj.onc.1207750 Published online 7 June 2004 [ABSTRACT FROM AUTHOR]