Our finding that T77‐phosphorylated IDH1 level was upregulated in cell line models of resistance and MPA‐refractory specimens indicates a role for T77‐phosphorylated IDH1 in cases where progestin resistance has occurred and support the future testing of T77‐phosphorylated IDH1 targeting in endometrial cancer patients with progestin resistance. Here, IDH1 is linked to endometrial cancer.