strong evidence in rats supports the conclusion that neonatal estrogen exposure plays a role in the development of prostate cancer. using a human fetal prostate xenograft model, researchers studied the effects of early exposure to estrogen with and without secondary estrogen and testosterone treatment. a xenograft model is a graft of tissue transplanted between organisms of different species. in this case, human tissue was transplanted into rats; therefore, there was no need to extrapolate from rodents to humans. histopathological lesions, proliferation, and serum hormone levels were measured at various time-points after xenografting. at day 200, the xenograft that had been exposed to two treatments of estrogen showed the most severe changes. additionally, researchers looked at key genes involved in prostatic glandular and stromal growth, cell-cycle progression, apoptosis, hormone receptors, and tumor suppressors using a custom pcr array. analysis of dna methylation showed methylation differences in cpg sites of the stromal compartment after estrogen treatment. these variations in methylation are likely a contributing cause to the changes in the cellular events in the kegg prostate cancer pathway that inhibit apoptosis and increase cell cycle progression that contribute to the development of cancer.