ERBB2 and breast cancer: Through non-negative matrix factorization (NMF) cluster analysis on 131 training samples, we identified four molecular subtypes for HR+/HER2+ breast cancer: MUKDEN I (n = 41), MUKDEN II (n = 38), MUKDEN III (n = 30), and MUKDEN IV (n = 22)—each with distinct gene expression profiles (Fig. 1b and Supplementary Fig. 1a).