expression of the digestive enzyme amylase. Also, the primary promoter for bile acid-CoA:amino acid N-acyltransferase (BAAT), which codes for an enzyme that is integral in bile metabolism, is of LTR origin. The insertion of a solo ERV-9 LTR may have produced a functional open reading frame, causing the rebirth of the human immunity related GTPase gene (IRGM). ERV insertions have also been shown to generate alternative splice sites either by direct integration into the gene, as with the human leptin hormone receptor, or driven by the expression of an upstream LTR, as with the phospholipase A-2 like protein. Most of the time, however, the LTR functions as one of many alternate promoters, often conferring tissue-specific expression related to reproduction and development. In fact, 64% of known LTR-promoted transcription variants are expressed in reproductive tissues. For example, the gene CYP19 codes for aromatase P450, an important enzyme for estrogen synthesis, that is normally expressed in the brain and reproductive organs of most mammals. However, in primates, an LTR-promoted transcriptional variant confers expression to the placenta and is responsible for controlling estrogen levels during pregnancy. Furthermore, the neuronal apoptosis inhibitory protein (NAIP), normally widespread, has an LTR of the HERV-P family acting as a promoter that confers expression to the testis and prostate. Other proteins, such as nitric oxide synthase 3 (NOS3), interleukin-2 receptor B (IL2RB), and another mediator of estrogen synthesis, HSD17B1, are also alternatively regulated by LTRs that confer placental expression, but their specific functions are not yet known. The high degree of reproductive expression is thought to be an after effect of the method by which they were endogenized; however, this also may be due to a lack of DNA methylation in germ-line tissues. The best-characterized instance of placental protein expression comes not from an alternatively promoted host gene but from a complete co-option of a retroviral protein. Retroviral fusogenic env proteins, which play a role in the entry of the virion into the host cell, have had an important impact on the development of the mammalian placenta. In mammals, intact env proteins called syncytins are responsible for the formation and function of syncytiotrophoblasts. These multinucleated cells are mainly responsible for maintaining nutrient exchange and separating the fetus from the mother's immune system. It has been suggested that the selection and fixation of these proteins for this function have played a critical role in the evolution of viviparity. In