ESR1 and breast cancer: Further in line with this, while ER- tumors are more proliferative than ER+ tumors, as measured by the expression levels of the S phase-specific proliferation marker PCNA (Fig. 3F,G), EEF1A1 mRNA-low tumors are more proliferative than EEF1A1 mRNA-high tumors in ER+ but not in ER- breast cancers (Fig. 3G).