Feedback control that prevents cells with incompletely assembled spindles from leaving mitosis. It interacts with the anaphase promoting complex/cyclosome (APC/C) thereby inhibiting APC/C-dependent proteolysis, a step required for exit from mitosis. Interacts with mad3 and slp1. Simultaneous disruption of nucleoporin alm1 results in a growth defect. Belongs to the MAD2 family.