ERBB2 and breast cancer: Gene expression profiling has allowed to classify breast cancer into four subtypes [3]: luminal A (ER- and/or PR-positive, HER-2-negative, Ki-67 expression less than 14%), luminal B (ER- and/or PR-positive, HER-2-negative, Ki-67 expression more than or equal to 14%; or ER- and/or PR-positive, HER-2 overexpression, Ki-67 at any level), basal-like (ER- and PR-negative, HER-2-negative), and HER-2 overexpression (ER- and PR-negative, HER-2 overexpression).