Microtubule fragments exhibiting net treadmilling in a Rac1(Q61L)-expressing PtK1 cell. Microtubule plus ends undergo dynamic instability while their minus ends depolymerize, resulting in net treadmilling of the microtubule lattice. Epifluorescence image series were acquired on an inverted microscope (model TE300; Nikon) equipped with electronically controlled shutters, filter wheels, and a 14-bit cooled CCD camera (Orca II; Hamamatsu Corporation) controlled by MetaMorph® software (Universal Imaging Corp.) at 5-s intervals using a 60/1.4 NA Plan Apo objective lens (Nikon). For still images, see Fig. 2 D of associated paper (J Cell Biol, 161:845-851, 2003)