Together, these two endocrine treatment regimens show that Nf1Indel-induced tumors highly depend on estrogen signaling for continued growth, but in the absence of substantial estrogen signaling such as under estrogen deprivation or fulvestrant treatment, the residual tumors can regain growth, mimicking the clinical behavior of a significant proportion of human ER+/PR+ breast cancer. Here, PGR is linked to breast cancer.