The steps upstream of spastin recruitment in AD and HSP might be similar in spite of the different outcomes: In case of pathological severing as in the proposed model for AD, microtubules first need to be polyglutamylated (by TTLL6 and possibly also by TTLL11 and the priming ligases TTLL1/5) [38]. The gene discussed is TTLL1; the disease is Alzheimer disease.