Primary breast tumors with a high Ki67 proliferation index, using a cut-off of 20% according to ESMO Early breast cancer guidelines [34], had higher GYS1 expression than Ki67-low tumors (median H-score 85, IQR 57–124 vs. 52, IQR 32–80; P < 0.0001; Fig. 1f, Suppl. Here, GYS1 is linked to breast cancer.