MAPK6 and breast cancer: Second, given the significant impact of the distinct molecular subtypes of breast cancer on metastatic rate and prognosis, ERK3 expression was further examined based on the three established clinical groups: oestrogen/progesterone receptor positive (ER/PR+), human epidermal growth factor receptor 2 positive (HER2+), and TNBC (ER/PR/HER2−) subtypes (39).