Each transcription factor acts in particular groups of cells. Therefore, various genetic mutations are associated with specific hormone deficiencies. For instance, "POU1F1" (also known as Pit-1) mutations cause specific deficiencies in growth hormone, prolactin and TSH. In addition to the pituitary, some of the transcription factors are also required for the development of other organs; some of these mutations are therefore also associated with specific birth defects. Most of the hormones in the anterior pituitary are each part of an "axis" that is regulated by the hypothalamus. The hypothalamus secretes a number of releasing hormones, often according to a circadian rhythm, into blood vessels that supply the anterior pituitary; most of these are stimulatory (thyrotropin-releasing hormone, corticotropin-releasing hormone, gonadotropin-releasing hormone and growth hormone-releasing hormone), apart from dopamine, which suppresses prolactin production. In response to the releasing hormone rate, the anterior pituitary produces its hormones (TSH, ACTH, LH, FSH, GH) which in turn stimulate effector hormone glands in the body, while prolactin (PRL) acts directly on the breast gland. Once the effector glands produce sufficient hormones (thyroxine, cortisol, estradiol or testosterone and IGF-1), both the hypothalamus and the pituitary cells sense their abundance and reduce their secretion of stimulating hormones. The hormones of the posterior pituitary are produced in the hypothalamus and are carried by nerve endings to the posterior lobe; their feedback system is therefore located in the hypothalamus, but damage to the nerve endings would still lead to a deficiency in hormone release. Unless the pituitary damage is being caused by a tumor that overproduces a particular hormone, it is the lack of pituitary hormones that leads to the symptoms described above, and an excess of a particular hormone would indicate the presence of a tumor. The exception to this rule is prolactin: if a tumor compresses the pituitary stalk, a decreased blood supply means that the lactotrope cells, which produce prolactin, are not receiving dopamine and therefore produce excess prolactin. Hence, mild elevations in prolactin are attributed to stalk compression. Very high prolactin levels, though, point more strongly towards a prolactinoma (prolactin-secreting tumor). The diagnosis of hypopituitarism is made on blood tests.