Endocrine therapy and CDK4/6 inhibition are the standard-of-care treatments for patients with advanced or metastatic estrogen receptor-positive (ER+) breast cancer. Despite improvements in the overall progression-free survival, resistance develops in patients by unidentified mechanisms. Preliminary CRISPR/Cas9 screens from the Caldon lab revealed that loss of the MAPK9 gene (encoding JNK2) …