From the seven different apolipoproteins identified in the sera of HER2-positive BC patients, five proteins were commonly identified in both responders and non-responders: apolipoprotein A-I (APOA1), apolipoprotein A-IV (APOA4), apolipoprotein B-100 (APOB), apolipoprotein E (APOE), and apolipoprotein M (APOM). This evidence concerns the gene APOA4 and breast cancer.