The classification recognizes lymphocyte-activated tumors (both ER− and ER+), carcinomas with a unique enrichment in HER2-related features (higher ERBB2 copy number, relative higher frequency of ERBB2 mutations), carcinomas with remodeling of the tumor-stromal interface, and a class with the most pronounced actionability of PIK3CA mutations. The gene discussed is ERBB2; the disease is neoplasm.