The molecular mechanisms of endocrine resistance in breast cancer remain poorly understood. Here we used PRO-seq to map the location of hundreds of genes and thousands of distal enhancers whose transcriptional activities differ between endocrine sensitive and resistant MCF-7 cells. Our genome-wide screen discovered increased transcription of the glial-cell line derived neurotrophic factor (GDNF), a RET tyrosine kinase receptor ligand, which we validate as both necessary and sufficient for resistance in MCF-7 cells. GDNF caused endocrine resistance by switching the active state of a bi-stable feedback loop in the MCF-7 regulatory network from ERα signaling to GDNF-RET signaling. To cause this switch, GDNF downregulated ERα transcription and activated the transcription factor EGR1, which, in turn, induced GDNF. Remarkably, both MCF-7 cells and ER+ primary tumors appear poised for endocrine resistance via the RET signaling pathway, but lack robust RET ligand expression and only develop resistance upon expression of GDNF or other RET ligands.