ESR1 and breast cancer: Interestingly, MATR3 expression was higher in estrogen receptor (ER)-positive, progesterone receptor (PR)-positive and HER2-negative breast cancers than in ER-, PR-, and HER2 + breast cancers (Fig. 6c–e), which consistently suggests that MATR3 expression is inversely correlated with aggressiveness and poor prognosis of breast cancer.