ESR1 and breast cancer: Aging female mice with a heterozygous Scurfy (sf) mutation of Foxp3 (Foxp3sf/+), which causes a loss of Foxp3 expression, frequently develop spontaneous breast cancers (ER+, 14/18; PR+, 12/18; ErbB2+, 18/18) after 1 year of age, and 40% of these mice with primary breast tumors also develop lung metastases [26].