PHGDH and breast cancer: To determine if PHGDH knockdown effects were due to global serine depletion, we increased the levels of serine or methyl-serine in cell culture medium, but this was insufficient to rescue PHGDH knockdown-mediated growth inhibition of BT-20 (Supplementary Fig.3), suggesting that ER- breast cancer cells depend on a specific function of PHGDH and glucose-derived serine and glycine production, or alternately that these cells rely on PHGDH to perform a function independent of serine and glycine production.