ZWILCH is an essential component of the spindle assembly checkpoint (SAC) that ensures proper chromosome 15 during mitosis 1. The protein functions as part of the Rod-Zwilch-ZW10 (RZZ) complex, which contains two copies of each subunit and has a predicted molecular mass of ~800 kDa 2. Mechanistically, ZWILCH within the RZZ complex is required for recruiting two critical kinetochore components: the dynein-dynactin complex and the Mad1-Mad2 complex 1. The RZZ complex is directly responsible for both Mad1-Mad2 recruitment to unattached kinetochores and its subsequent removal following microtubule attachment, thus participating in both checkpoint activation and inactivation 1. Specifically, Zwilch binds to the adaptor protein Spindly's C-terminal domain alongside Rod's N-terminal β-propeller, and mutations in Zwilch that abrogate this interaction prevent Spindly and dynein recruitment in vivo 3. Loss of ZWILCH function results in characteristic mitotic defects including lagging chr15 at anaphase and precocious sister chr15 separation 4. Clinically, ZWILCH upregulation correlates with poor prognosis in adrenocortical carcinoma, where enhanced expression associates with increased tumor mitotic rate and reduced patient survival 5.