+331G/A, creates a unique transcription start site. Biochemical assays showed that the +331G/A polymorphism increases transcription of the PR gene, favoring production of hPR-B in an Ishikawa endometrial cancer cell line. Several studies have now shown no association between progesterone receptor gene +331G/A polymorphisms and breast or endometrial cancers. However, these follow-up studies lacked the sample size and statistical power to make any definitive conclusions, due to the rarity of the +331A SNP. It is currently unknown which if any polymorphisms in this receptor are of significance to cancer. A study of 21 non-European populations identified two markers within the PROGINS haplotype of the PR gene as positively correlated with ovarian and breast cancer. Animal studies Development Knockout mice of the PR have been found to have severely impaired lobuloalveolar development of the mammary glands as well as delayed but otherwise normal mammary ductal development at puberty. Behavior During rodent perinatal life, progesterone receptor (PR) is known to be transiently expressed in both the ventral tegmental area (VTA) and the medial prefrontal cortex (mPFC) of the mesocortical dopaminergic pathway. PR activity during this time period impacts the development of dopaminergic innervation of the mPFC from the VTA. If PR activity is altered, a change in dopaminergic innervation of the mPFC is seen and tyrosine hydroxylase (TH), the rate-limiting enzyme for dopamine synthesis, in the VTA will also be impacted. TH expression in this area is an indicator of dopaminergic activity, which is believed to be involved in normal and critical development of complex cognitive behaviors that are mediated by the mesocortical dopaminergic pathway, such as working memory, attention, behavioral inhibition, and cognitive flexibility. Research has shown that when a PR antagonist, such as RU 486, is administered to rats during the neonatal period, decreased tyrosine hydroxylase immunoreactive (TH-ir) cells density, a strong co-expresser with PR-immunoreactivity (PR-ir), is seen in the mPFC of juvenile rodents. Later on, in adulthood, decreased levels of TH-ir in the VTA are also shown. This alteration in TH-ir fiber expression, an indicator of altered dopaminergic activity resulting from neonatal PR antagonist administration, has been shown to impair later performance on tasks that measure behavioral inhibition and impulsivity, as well as cognitive flexibility in adulthood. Similar cognitive flexibility impairments were also seen in PR knockout mice as a result of reduced dopaminergic activity in the VTA. Conversely, when a PR agonist, such as 17α-hydroxyprogesterone caproate, is administered