ESR1 and breast cancer: No significant differences in the levels of transcripts of isoforms 2, 3 and 4 were detected in this study, although overexpression of SDC4 has been previously described for an estrogen receptor-negative highly proliferative breast carcinoma subtype [28]; however, most samples analyzed in this study displayed a luminal A phenotype, which constitutes a common early stage of breast cancer where tumors have higher levels of estrogen and progesterone receptors.