Gene fusions play a critical role in cancer progression. The mechanisms underlying their genesis and cell type specificity are not well understood. About 50% of human prostate cancers display a gene fusion involving the 5' untranslated region of TMPRSS2, an androgen-regulated gene, and the protein-coding sequences of ERG, which encodes an erythroblast transformation-specific (ETS) transcription factor. By studying human prostate cancer cells with fluorescence in situ hybridization, we show that androgen signaling induces proximity of the TMPRSS2 and ERG genomic loci, both located on chromosome 21q22.2. Subsequent exposure of the cells to gamma irradiation, which causes DNA double-strand breaks, facilitates the formation of the TMPRSS2-ERG gene fusion. These results may help explain why TMPRSS2-ERG fusions are restricted to the prostate, which is dependent on androgen signaling.