The authors respond to Roy Burns' recent review on tubulin tyrosination, highlighting a gap in his discussion of recent relevant studies. They argue that while detirosinated (Glu) tubulin protomers are not polymerized into microtubules in vivo due to their rapid retrotyrosination, this does not mean they are incompetent for polymerization. Instead, the absence of Glu protomers in the cytoplasm is due to their rapid conversion back to tyrosinated (Tyr) form. The authors propose a cycle involving the polymerization of Tyr protomers, detirosination of Tyr microtubules, breakdown of Glu-enriched microtubules, and efficient retirosination of Glu protomers. They suggest that the gradient of Glu/Tyr ratios along microtubules would depend on the rates of polymerization and detirosination. The longevity of Glu-enriched microtubules and the relationship between Glu tubulin levels and microtubule stability remain unclear. The authors conclude that further research is needed to elucidate the mechanisms of these post-translational modifications, which may involve specific microtubule-associated proteins recognizing Glu tubulin to influence microtubule function or interactions.The authors respond to Roy Burns' recent review on tubulin tyrosination, highlighting a gap in his discussion of recent relevant studies. They argue that while detirosinated (Glu) tubulin protomers are not polymerized into microtubules in vivo due to their rapid retrotyrosination, this does not mean they are incompetent for polymerization. Instead, the absence of Glu protomers in the cytoplasm is due to their rapid conversion back to tyrosinated (Tyr) form. The authors propose a cycle involving the polymerization of Tyr protomers, detirosination of Tyr microtubules, breakdown of Glu-enriched microtubules, and efficient retirosination of Glu protomers. They suggest that the gradient of Glu/Tyr ratios along microtubules would depend on the rates of polymerization and detirosination. The longevity of Glu-enriched microtubules and the relationship between Glu tubulin levels and microtubule stability remain unclear. The authors conclude that further research is needed to elucidate the mechanisms of these post-translational modifications, which may involve specific microtubule-associated proteins recognizing Glu tubulin to influence microtubule function or interactions.