In the randomized EMILIA trial, which compared T-DM1 and capecitabine-lapatinib (CL) for pretreated metastatic HER2+ BC patients, patients with tumor ERBB2 mRNA levels above median showed a greater benefit from the use of T-DM1 in terms of ORR and overall survival (OS). The gene discussed is ERBB2; the disease is breast cancer.