The luminal status of the breast cancer is then combined with the expression status of the erb-b2 receptor tyrosine kinase (ERBB2) gene, a proto-oncogene which encodes for the human epidermal growth factor receptor 2 (HER2) protein, in order to determine if an overabundance of HER2 plays a role in tumour progression and to classify the cancer into one of four specific groups: luminal A (group 1), luminal B (group 2), HER-2 positive (group 3), and triple negative (group 4) [7]. Here, ERBB2 is linked to cancer.