Excisional biopsy of the breast revealed an intraductal papilloma, while video-assisted thoracoscopic biopsy of the lung nodules demonstrated benign smooth muscle proliferation positive for α-smooth muscle actin and estrogen receptor (ER), with a very low Ki-67 index, findings consistent with BML. This case illustrates the diagnostic challenge of distinguishing BML from metastatic malignancy in patients with concurrent lesions. Here, ESR1 is linked to intraductal papilloma.