ESR1 and breast cancer: In normal human breast tissue, the 5′UTRs of the ERα transcripts are predominantly from exons C and A with the expression of transcript with exon A greater than the transcript with exon C. In human breast tumors and breast cancer cells, transcripts with exons C, B, and A are overexpressed with the expression of the transcript with exon C greater than the transcript with exon A [83,84,85].