Before revision:  Recent in vitro {\it  observations by Gardner et al.[  Cell {\bf147}, 1092 (2011)] showed that microtubule catastrophe takes place via multiple steps , and the frequency increases with the "age " of the filament.
After revision:  Recent in vitro {\it observations by Gardner et al.[ M. K. Gardner et al., Cell {\bf147}, 1092 (2011)] showed that microtubule catastrophe takes place via multiple steps , and the frequency increases with the "age " of the filament.