it might even prove lethal when in a yeast background considering the improbable nature of any effect a screen assay was developed for screening hundreds of thousands of transformants remarkably in the first series of experiments a single active clone clone 1 was identified out of a total of 200 000 transformants sequence analysis revealed significant homology of the human cdna to the cdc34 gene of s cerevisiae clone 1 human cdna efficiently complements a yeast cdc34 mutation identifying it as a human homolog of yeast cdc34 the result of this assay was particularly unexpected since all previous reports linked the function of yeast cdc34 to events in the late g1 phase of the cell cycle prior to entry into s phase interestingly the limited sequence homology between yeast and man was not sufficient for yeast cdna to identify human cdc34 s plon unpublished as used herein 34 checkpoint 34 is intended to mean a timepoint in the cell cycle of a eukaryotic cell at which progression to mitosis may be arrested if the cell contains chromosomal dna with one or more strand breaks illustrative methods by which dna strand breaks may be introduced into chromosomal dna include dna ligase mutations topoisomerase mutations x irradiation gamma irradiation and treatment with drugs e g hydroxyurea or treatment with chemotherapeutic.