• First crystal structures of WDR5 bound to Kif2A-derived peptides reveal a dual engagement of both WIN and S7 sites, with Arg117 and Ser121 as key anchors.
• Ser121 induces a conformational change in Tyr191, opening the S7 pocket, which is critical for high-affinity binding and inhibitor mimicry.
• Mutagenesis and ITC confirm the functional importance of Arg117 and Ser121, providing a molecular basis for the non-canonical mitotic role of WDR5.
• The WIN-S7 site plasticity offers a promising therapeutic target for cancers linked to chromosomal instability, guiding dual-site inhibitor design.