Seven studies reported the prevalence of HER2-low tumors among all BC (0, 1+, 2+/ISH-, 2+/ISH+, and 3+) [20–23, 27–29], while the other five studies reported the prevalence of HER2-low among HER2-negative cases (0, 1+, and 2+/ISH-) [18, 19, 24–26], with sample sizes ranging from 55 to 3,737 and from 44 to 62,985 individuals, respectively. This evidence concerns the gene ERBB2 and breast cancer.