The combined biochemical and structural results show that the S280F substitution enables resistance to ivosidenib in cancer cells producing R132C/S280F or R132H/S280F in part by hindering its binding to the IDH1 dimer-interface and in part by increasing the efficiency of R132C and R132H variants in catalysing conversion of isocitrate and/or 2-OG to 2-HG. Here, IDH1 is linked to cancer.