The structure of hus1+, a Schizosaccharomyces pombe gene required for S-M and DNA damage checkpoints, has been determined. Expression of hus1+ requires splicing of five exons, including a microexon that is only 13 nucleotides long. hus1+ is predicted to encode a 33 kDa protein with no similarity to sequences in any database, including the entire S. cerevisiae genome. Yeast strains disrupted for the hus1+ gene are viable but checkpoint-defective. Polyclonal antibodies were raised against bacterially expressed Hus1 protein, and used to study Hus1 regulation. Hus1 protein levels are not affected by S-phase arrest, and are not altered by mutations in other checkpoint genes, suggesting that Hus1 is not regulated at the transcriptional or translational levels.