42 KRpep‐2d inhibited cancer cell proliferation at 30 μm, which is high compared to the in vitro IC50 value. The crystal structure of KRas G12D in complex with GDP and KRpep‐2d was obtained, confirming that KRpep‐2d bound to a cleft near the switch II region.43 KRpep‐2d acts as an allosteric inhibitor, stabilizing the switch II region (distal to Sos‐binding site) in a conformation non‐conducive to nucleotide exchange. This evidence concerns the gene KRAS and cancer.