Expression data were downloaded from the following websites:

• Human: Whitfield et al. 2002
• Budding Yeast: Spellman et al. 1998
• Fission Yeast:
  • Rustici et al. 2004 (407 cell cycle genes by Rustici et al.)
  • Oliva et al. 2005 (750 cell cycle genes by Oliva et al.)
  • Peng et al. 2005 (747 cell cycle genes by Peng et al.)
• Arabidopsis: Menges et al. 2002