mESCs were cultured at least 2 weeks in medium medium: N2B27 for SILAC (DMEM/F12 for SILAC (AthenaES), Neurobasal for SILAC (AthenaES), Sodium Pyruvate (40 mg/mL), N2 (1X), B27 (0.5X), Pen/Strep (1%), L-glutamine (2 mM), beta-mercaptoethanol (50 µM)) + 2i/LIF supplemented with (medium) 13C6 15N4 L-arginine (0.65 mM), (medium) 13C6 L-arginine (0.55 mM), and L-Proline (200 mg/L)84. Before switching from light to heavy medium, cells were treated with 0.05 µg/mL CHX or equivalent dilution of DMSO in 6-well plates. We treated the cells with 0.05 µg/mL CHX. At timepoint 0 h, we replaced medium medium with pre-warmed heavy medium: N2B27 for SILAC (Thermo scientific) supplemented with (heavy) 13C6 15N2 L-Lysine-2HCl (0.65 mM), (heavy) 13C6 15N4 L-Arginine-HCl (0.55 mM), and L-Proline (200 mg/L) for mESC.