General view of the species-specific regulation of histone homeostasis. In mammals (left) , histone transcription is activated by NPAT (Nuclear protein, ataxia-telangiectasia locus) and SLBP is then bound to the 3' end of histone mRNA, by which it prevents degradation of mRNAs, resulting in synthesis of histone proteins. At the end of S phase, CDK1-cyclinA (cycA) phosphorylates SLBP to trigger its degradation, restraining further transcription of histone mRNAs. In fission yeast (middle) , Ams2 activates histone transcription at G1/S phase. At the S/G2 phase, Ams2 is phosphorylated by DDK, leading to degradation via the SCFPof3-ubiquitin proteasome pathway. In budding yeast (left) , excess histones are recognised and phosphorylated by Rad53. The histone-Rad53 complex is recognised by the Ubc4/5 (E2) and Tom1 (E3) and is polyubiquitylated. Histones with a polyubiquitin chain are degraded by the proteasome.