Transgenic mice recapitulating TMPRSS2-ERG in the prostate develop mPIN. (a) To recapitulate TMPRSS2-ERG in vivo, we generated transgenic mice over-expressing the ERG gene fusion product (exons 2 through the reported stop codon; 1533 of NM_182918.2, C-terminal 3X-FLAG epitope tag) with a bovine growth hormone polyA signal (PA-BGH) under the control of the enhanced probasin promoter (ARR2Pb). Mice were sacrificed at 12 to 14 weeks or >20 weeks, and mouse prostatic intraepithelial neoplasia (mPIN) was observed in 4 of 11 ARR2Pb-ERG mice as described in Table W1. Benign epithelia and areas of mPIN are indicated by yellow and black arrows, respectively. (b–d) Hematoxylin and eosin staining of ARR2Pb-ERG prostates for morphologic assessment. Consistent with the focality of mPIN, (b) benign glands and (c and d) mPIN were observed in the ventral prostate (VP) of ARR2Pb-ERG mice. Original magnification: (b) ×400, (c) ×200, and (d) inset showing area of mPIN with macronucleoli, ×400.