With the advent of gene targeting in pluripotent mouse embryonic stem cells, it is now possible to modify the mammalian genome to generate mutant strains of mice with precise genetic mutations. The major goal of my laboratory is to generate transgenic mice to use as physiologic models to study mammalian reproduction and development. The initial focus of our research has been to generate mice deficient in inhibins, activins, activin binding proteins (i.e., follistatin), and activin receptors (i.e., activin receptor type II) to understand their interactions and roles in the hypothalamic-pituitary-gonadal axis and mammalian development. Inhibins and activins, dimeric members of the TGF-beta superfamily, were discovered due to their role in pituitary follicle stimulating hormone homeostasis. However, these proteins have later been shown to have diverse endocrine, paracrine, and autocrine functions. Activins have been shown to mediate their signals through type I and type II serine/threonine kinase receptors. The high interspecies conservation of activins, inhibins, and activin receptors and the universal presence of activins in mammals, birds, amphibians, and fish suggest an evolutionarily conserved role of these proteins in animal development. Our initial studies have demonstrated a tumor suppressor role of inhibin in the gonads and adrenals and have also suggested a role of activins in cancer cachexia-like syndrome. To further study the gonadal tumor development and the cancer cachexia-like syndrome in these mice, we have begun to generate mice with multiple genetic alterations (e.g., mice deficient in both inhibin and Mullerian inhibiting substance). We have also generated mice deficient in other components of this complex system (e.g., activin beta A, activin receptor type II, follistatin). Analysis of these transgenic mutant models has aided our overall understanding of the critical roles these proteins play in the development of the reproductive system, in the modulation of the endocrine milieu that regulates reproductive function, and in mammalian development.