XMAP215 promotes microtubule growth rate fluctuations and tapered microtubule ends. Growth rate–matching conditions achieved by either 60 µM tubulin and 200 nM EB1-GFP (condition I) or 20 µM tubulin, 200 nM EB1-GFP, and 12.5/25 nM XMAP215 (condition II). (A) SSR was determined from 10-s segments that only displayed full comets. Left and center: Representative tracks showing microtubule tip position (black points), residuals for each time point (black lines), and linear regression to tip position (red line). Right: SSR for each segment; condition I, 0.013 ± 0.007 µm²/s (mean ± SD, n = 103); condition II, 0.02 ± 0.01 µm²/s (n = 90). ****, P < 0.0001, unpaired Welch’s t test. (B) An example microtubule with distinct EB1 comet morphologies: full, split, and curled. Intensity profiles along the dashed line at indicated time points. (C) Classification of 110 growth events into comet morphology categories. Condition I: 96 full, 13 split, 1 curled; condition II: 25 full, 37 split, 48 curled. ****, P < 0.0001, χ² test.