Globular tubulin subunits comprise the microtubule building block, with a/b-tubulin heterodimers forming the tubulin subunit common to all eukaryotic cells. As a critical part of the microtubule-organizing center (MTOC), the third member of the tubulin superfamily, y-tubulin, is required for microtubule nucleation as well as centrosome duplication and spindle assembly (1,2, reviewed in 3). y-tubulin forms complexes of two different sizes: y-tubulin small complex (y-TuSC) and the larger y-tubulin ring complex (y-TuRC). Each complex consists of a number of y-tubulin complex proteins (GCPs) with y-tubulin itself being considered GCP1. GCP2-6 all share sequence similarity in 5 different regions and it is thought that these areas could play a role in the proper folding of the proteins (4). y-TuSC is composed of two y-tubulin molecules as well as GCP2 and GCP3. y-TuRC is made up of a ring of multiple copies of y-TuSC in addition to GCP4, 5, and 6. Another protein, GCP-WD/NEDD1, which lacks sequence similarity with the other GCPs, associates with the y-TuRC. GCP-WD/NEDD1 has been shown to regulate localization of the y-TuSC to spindles and centrosomes (5-8). In mammals, phosphorylation of y-tubulin at Ser131 by SADB controls the activity of the y-TuRC. The hypothesis is that this phosphorylation stabilizes the protein in a conformation that stimulates centrosome amplification (9).