ERBB2 and breast cancer: Among women, breast cancer is the most commonly diagnosed cancer and the leading cause of cancer-related death globally.1 In the US over the past decade from 2010 to 2019, breast cancer incidence rates have increased 0.5% annually.2 However, breast cancer is a heterogeneous disease consisting of 4 main subtypes defined by joint hormone receptor (HR) and ERBB2 status: HR-positive and ERBB2-negative, HR-negative and ERBB2-negative, HR-positive and ERBB2-positive, and HR-negative and ERBB2-positive.