ESR1 and breast cancer: The glutamate/glutamine ratio (GGR) correlates with estrogen receptor (ER) status in breast cancer (“glutamate enrichment” being found in 56 % of ER+ and 88 % of ER− breast cancer, opposing 2.2 % of normal breast tissue), with tumor grading (GGR as a MYC-associated marker of aggressive disease), and with tyrosine metabolism [43].