Estradiol influences these processes by binding to estradiol receptors (ER). They are ligand-depending transcription-factors. In humans there are two subtypes: ERa and ERb, which are synthetized from gene ESR1 on chromosome 6 and ESR2 on chromosome 14. Both subtypes are expressed in the ovary. Both genes are polymorph. Especially for ERa subtype several polymorphisms and mutations are known which can be linked to breast cancer, spontaneous abortions, osteoporosis and the point in time of menarche. Furthermore some studies have shown a relationship between certain polymorphisms and the risk of infertility associated gynecological malfunctions and the result of IVF treatments.