Premenopausal status If premenopausal, patient must be willing to comply with pregnancy requirements PIKCA MUTANT COHORT (closed //): In premenopausal women, serum estradiol level in postmenopausal range =< days prior to registration PIKCA WILD TYPE COHORT (closed //): If premenopausal, patient must be willing to comply with pregnancy requirements PIKCA WILD TYPE COHORT (closed //): In premenopausal women, serum estradiol level in postmenopausal range =< days prior to registration ENDOCRINE RESISTANT COHORT: Pre- or post-menopausal women are eligible; if premenopausal, patient must be willing to comply with pregnancy requirements and agrees with GnRH agonist therapy for ovarian suppression during the study ADJUVANT COHORT: If premenopausal, patient must be willing to comply with pregnancy requirements Patients can be premenopausal or postmenopausal Premenopausal status is defined as either: I). Patient had last menstrual period within the last months, OR ii). If on tamoxifen or toremifene within the past days, plasma estradiol and FSH must be in the premenopausal range per local normal range, OR iii). In case of therapy induced amenorrhea, plasma estradiol and/or FSH must be in the premenopausal range per local normal range. Perimenopausal status is define as neither premenopausal nor postmenopausal Note: Throughout this document, perimenopausal and premenopausal status is grouped together and referred as \Premenopausal\ Patient is premenopausal defined as either: If on tamoxifen or toremifene with in the past days, plasma estradiol and FSH must be in the premenopausal range per local normal range, or In case of therapy induced amenorrhea, plasma estradiol and/or FSH must be in the premenopausal range per local normal range Patient is premenopausal or perimenopausal at the time of study entry Premenopausal status Premenopausal status Premenopausal or peri-menopausal women. Premenopausal levels of estradiol, or ongoing menses Study subjects should be post-menopausal women (premenopausal women are eligible if they are on or willing to be on mandatory ovarian function suppression) Women for whom adjuvant treatment with an aromatase inhibitor would be clinically indicated; women must be either post-menopausal, or premenopausal having undergone oophorectomy Systemic drug treatment to induce ovarian suppression if woman is premenopausal In premenopausal women, serum estradiol level in postmenopausal range =< days prior to registration Premenopausal or postmenopausal women Female patient is pregnant, breastfeeding, or premenopausal and not using an effective method of contraception Patient must be premenopausal at breast cancer diagnosis, as determined locally and documented in patient record; (Note: it is understood that patients menopausal status may be unclear at the time of study enrollment) Premenopausal Premenopausal Premenopausal women