Hus1, Rad1, and Rad9 are three evolutionarily conserved proteins required for checkpoint control in fission yeast. These proteins are known to form a stable complex in vivo [1]. Hus1-Rad1-Rad9 complex may form a PCNA-like ring structure, and could function as a sliding clamp during checkpoint control.

1: Structure-function analysis of fission yeast Hus1-Rad1-Rad9 checkpoint complex. Kaur R, Kostrub CF, Enoch T; Mol Biol Cell 2001;12:3744-3758. PMID:11739777

2: Yeast pip3/mec3 mutants fail to delay entry into S phase and to slow DNA replication in response to DNA damage, and they define a functional link between Mec3 and DNA primase. Longhese MP, Fraschini R, Plevani P, Lucchini G; Mol Cell Biol 1996;16:3235-3244. PMID:8668138

3: Characterization of Schizosaccharomyces pombe Hus1: a PCNA-related protein that associates with Rad1 and Rad9. Caspari T, Dahlen M, Kanter-Smoler G, Lindsay HD, Hofmann K, Papadimitriou K, Sunnerhagen P, Carr AM; Mol Cell Biol 2000;20:1254-1262. PMID:10648611