Human stathmin was cloned into the NH 2 terminal His-tagged pReceiver-M01 expression vector (Genecopoeia, Rockville, MD). Construction of mutant stathmin cDNAs, where the codons for Ser-16, Ser-25 or Ser-38 are exchanged to Ala, was performed by site-directed mutagenesis using a QuikChange kit (Stratagene, La Jolla, CA) following the manufacturer's instructions. Primers used, with the introduced mutations underlined, were: (Ser16RAla) 59-AGCGTGCCGCAGGCCAG-39; (Ser25RAla) 59-GCTGATT-CTCGCCCCTCGGTC-39; (Ser38RAla) 59-CCCCCTTGCC-CCTCCAAAG-39. All mutant constructs were confirmed by DNA sequence analysis.