ERBB2 and breast cancer: Before breast cancer surgery, the first principal component allowed us to distinguish luminal A and luminal B HER2-negative subtypes by the contribution of protein (r = 0.6820), LDH (r = 0.6690), urea (r = 0.6689) and catalase (r = 0.5498), the second principal component—by the content of α-amino acids (r = 0.7516).