Central component of the spindle assembly checkpoint which is a feedback control that prevents cells with incompletely assembled spindles from leaving mitosis. Thought to be recruited to unattached kinetochores by MAD1. During checkpoint activity; MAD2 is relayed from the MAD1-MAD2 complex to the mitotic checkpoint complex (MCC). MCC and presumably the MAD2-CDC20 subcomplex inhibit the ubiquitin ligase activity of the anaphase promoting complex/cyclosome (APC/C) by preventing its activation by CDC20. {ECO:0000269|PubMed:15879521; ECO:0000269|PubMed:8232541}.