Introduction
The glycoprotein hormone α-subunit (αSU) is the common element in a family of heterodimeric glycoprotein hormones which includes luteinising hormone (LH), follicle-stimulating hormone (FSH), thyroid-stimulating hormone (TSH) and human chorionic gonadotrophin (Pierce & Parsons 1981). The gene encoding the αSU exists as a single copy gene expressed in pituitary thyrotrophs and gonadotrophs of all mammals studied, but placental expression has only been found in primates and horses (Fiddes & Goodman 1981, Fiddes & Talmadge 1984, Fox & Solter 1988, Fenstermaker et al. 1990). During ontogeny, unique pituitary cell types differentiate in a sequential pattern and begin to synthesise and secrete specific hormones (Simmons et al. 1990, Borrelli 1994). The anterior pituitary develops from an invagination of the pharyngeal epithelium to form Rathke's pouch, and in situ hybridisation studies in the mouse have detected αSU mRNA in this region as early as embryonic day 11·5 (Simmons et al. 1990). At
