LIF is supposed to be one of the most important factor s involving mediating embryo implantation.However expression of LIF and its rece ptor varied among species.Further ,regulatory effects of progesterone and estrog en on secretion of LIF showed in different manners not only among species but al so between in vivo and in vitro,even individuals of the same species.The mechani sm of regulation of LIF on embryo implantation is drawing more research interest than before.