ESR1 and breast neoplasm: 17,20 Tumors that are naturally auxotrophic for serine, including luminal/estrogen-receptor-positive (ER+) breast tumors,20 are prime candidates for serine deprivation therapy.17,21–23 While dietary serine starvation is a promising therapeutic approach, long-term systemic serine deficiency has the potential to cause peripheral neuropathy.24–29 It has also recently been observed that stromal cells in the tumor microenvironment can secrete serine for uptake by auxotrophic cancer cells, which could also limit the efficacy of serine starvation therapy.23