ESR1 and breast cancer: Mammary tumors can be considered as hormone-dependent or hormone independent tumors based on their hormonal requirements for growth.[17] Estrogen receptor (ER) expression has been reported in N-nitroso-N-methylurea (NMU) induced rat mammary carcinomas[18, 19] although numerous reports have shown that breast cancer that initially responds to hormone therapy often develops into a hormone-independent type.