Long microtubules correlate with a higher percentage of MAD1-positive kinetochores. HeLa cells were transfected with EGFP-MAD1 and the indicated siRNA for 24 h before monastrol treatment. (A) Mean spindle length at 20 min after monastrol release for cells treated with MCAK-specific siRNA or control siRNA. N equals cell count. P = 0.01. (B) Percentage of Mad1-positive kinetochores (KTs) in cells treated as in A. (A and B) Error bars indicate SEM. (C) Spindles of different lengths showing kinetochores and the presence of MAD1 in cells treated as in A. Bar, 10 µm. (D) A model illustrating that in bipolar cells with long microtubules, free microtubule ends cross the metaphase plate and thus bypass many kinetochores so those kinetochores have more difficulty forming stable attachments to microtubules. As a result, MAD1 remains on these kinetochores.