While serine-auxotrophic cell lines require uptake of exogenous serine, non-auxotrophic cells can synthesize serine de novo and are less sensitive to low-serine conditions.20 To determine whether ASCT2 contributes to serine uptake in non-auxotrophic cell lines, we knocked out ASCT2 in the basal/triple-negative breast cancer cell lines HCC1806 and SUM149 that we have previously shown to be non-auxotrophic for serine20 (Figures 2A and S2A). Here, SLC1A5 is linked to triple-negative breast carcinoma.