Many breast cancers are steroid hormone dependent, and estrogen and androgen signaling have been shown to be the primary sex steroid hormones involved in regulating tumor growth and progression, with 70-80% of all breast cancers expressing estrogen receptor (ER)α [10, 11] and 60-80% expressing the androgen receptor (AR) [12, 13]. The gene discussed is ESR1; the disease is breast cancer.