ESR1 and breast cancer: Two breast cancer cell lines, MCF-7, which is estrogen-receptor positive and mimics an early stage of breast cancer, and MDA-MB-231, which is estrogen-receptor negative and mimics a late stage in breast cancer progression, were exposed to a 15-nucleotide sequence around the start codon of GLUT5, used as an antisense oligonucleotide to specifically block GLUT5 expression.