Disorders of the prostate and lower urinary tract are common in elderly men. We investigated the role of metallothionein-1 (MT1) in prostate carcinogenesis by generating a prostate-specific, MT1-expressing mouse. Unexpectedly, genomic analyses revealed that a 12.1-kb genomic region harboring several conserved noncoding elements was unintentionally deleted, upstream of the transgene integration site in the mouse, which we named it 12.1DMT1. Male 12.1DMT1 mice chronically treated with testosterone (T) plus 17b-estradiol (E2) to induce prostate cancer exhibited no evidence of precancerous or cancerous lesions. Instead, most of them exhibited a bladder outlet obstruction (BOO) phenotype not observed in treated wild-type (WT) mice. Thus, we hypothesized that 12.1DMT1 is a novel model for studying the hormonal requirement for BOO induction. Adult male 12.1DMT1 and WT mice were treated with T, E2, bisphenol A (BPA), T þ E2, or T þ BPA for up to 6 months. Histologic and immunohistochemical analysis of the prostate, bladder, and urethra were performed. No significant prostate pathologies were observed in WT or 12.1DMT1 mice treated with any of the hormone regimens. As expected, prostatic regression occurred in all E2-treated animals (WT and 12.1DMT1). Of great interest, despite a small prostate, 100% of E2-treated 12.1DMT1 mice, but only 40% of E2-treated WT mice, developed severe BOO (Po0.01). In contrast, T þ E2 treatment was less effective than E2 treatment in inducing severe BOO in 12.1DMT1 mice (68%, Po0.05) and was completely ineffective in WT animals. Similarly, T, BPA, and T þ BPA treatments did not induce BOO in either WT or 12.1DMT1 mice. The BOO pathology includes a thinner detrusor wall, narrowing of bladder neck and urethral lumen, and basal cell hyperplasia in the bladder body and urethra. These findings indicate that 12.1DMT1 mice exhibit enhanced susceptibility to E2-induced BOO that is independent of prostate enlargement but that is attenuated by the conjoint treatment with T. (2015) 95, 546-560; doi:10.1038/labinvest.2015 published online 23 February 2015 Bladder outlet obstruction (BOO) is regarded as a major cause of bothersome lower urinary tract symptoms (LUTS) in men. LUTS encompass a range of urinary morbidities, including those of storage, voiding, and postmicturition. 1 The incidence and prevalence of LUTS increase with age. 2 The etiologies of LUTS are multifactorial, and the pathophysiology is not well understood. 1,3 LUTS has often been attributed to benign prostatic hyperplasia (BPH), which narrows the urethral lumen and ultimately leads to BOO. However, a number of studies have suggested that BOO is not always caused by BPH. [4][5][6] Factors or organs other than the prostate have been proposed as contributors to BOO or modifiers of the severity of its symptoms. 3,7 An imbalance between androgens and estrogens has been implicated in the development of LUTS in men, including BOO. Higher levels of circulating 17b-estradiol (E2) were associated with an increased risk of LUTS in older men,...