The human placenta is a highly invasive tumor-like structure in which a subpopulation of placental trophoblast cells known as the "extravillous trophoblast" (EVT) invades the uterine decidua and its vasculature to establish adequate fetal-maternal exchange of molecules. In vitro-propagated short-lived EVT cell lines such as JEG-3 cells are used to study molecular mechanisms responsible for this invasiveness, which are identical to those of cancer cells; however, unlike cancer cells, their proliferation, migration, and invasiveness in situ are stringently controlled by decidua-derived transforming growth factor (TGF)-beta. These epithelial-trophoblast-like cells are therefore used in identifying genetic changes underlying epithelial tumor progression as well as gonadotrophin secretion studies.