Superovulation in horses is much less successful than in other domestic species. In addition to superovulation rate, the number of embryos are typically negligible (1). Hence, there is a great demand to develop products that induce multiple ovulations in mares due to changes in the governance of the horse reproduction industry (2). Follicle stimulating hormone (FSH), a member of the glycoprotein hormone family, plays an essential role in this process (3). Generally, the anterior pituitary gland is the tissue that is responsible for releasing this hormone (4-8), and granulosa cells in the ovary are the target cells for the numerous actions of FSH (6, 8-11). Similar to other members of the glycoprotein hormone family, including luteinizing hormone (LH), chorionic gonadotropin hormone (CGH), and thyroid stimulating hormone (TSH), FSH consists of two heterodimer non-covalently linked subunits: a common alpha subunit and a hormone-specific beta subunit (6, 12). An increase in the exogenous FSH concentration in the circulatory system during follicular development may leads to increase in the number of ovulatory follicles, which results in an increase in the number of mature follicles, the ovulatory rate, and the rate of embryo transfer (3). Currently, this method is applicable for both human infertility and animal breeding programs(13). Some gonadotropin sources with FSH-like activity, such as pituitary extraction from domestic animals, pregnant mare serum gonadotropin (PMSG), and FSH extraction from the urine of postmenopausal women, have been used for the same purpose. Despite the success of some of these treatments in many domestic species, these sources are less effective in mares (14-18). It has been reported that recombinant eFSH (reFSH) shows a much more specific effect in increasing the number of total preovulatory follicles than other therapeutic methods (3). Many of the recombinant proteins that have been successfully produced using the yeast expression system have been used in basic laboratory research and the therapeutic industry (19). The methylotrophic P. pastoris yeast is a suitable host for the production of many recombinant proteins because of its easy genetic manipulation, expression of intracellular or extracellular foreign protein (20), and capability to perform eukaryotic protein modifications, such as glycosylation, disulfide bond formation, and proteolytic processing (20-22).

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