This subtype of cancer is further subdivided into three subgroups, namely, HER2-positive if the score of immunohistochemistry is 3+ or if the score is 2+ and in situ hybridization is positive, HER2-low if the score of immunohistochemistry is 2+ and the in situ hybridization assay is negative or if the score of immunohistochemistry is 1+, and HER2-negative if the score of immunohistochemistry is 0 [82]. The gene discussed is ERBB2; the disease is cancer.