ESR1 and invasive breast carcinoma: Based on data that oophorectomy before the age of 40 years reduces the risk of invasive breast cancer development by approximately 60% [7], and solving for the number of ER- and ER+ tumors in the BCPT that emerged with and without therapy (Table 2), the transformation rate, Z, required to fit the BCPT data is equal to 20.1%.