To confirm that the RAD9A (first found as a gene that promotes G2 cell cycle arrest) is necessary to invoke G2 cell cycle arrest, they tested a double mutant of RAD9A with DNA repair deficient-strain and found that the cell failed to arrest in G2, further proving that a functional Rad9 gene is needed to induce G2 arrest.
Rad9 was first found as a gene that promotes G2 cell cycle arrest in response to DNA damage in Saccharomyces cerevisiae by Weinert et al. From this they suspected that Rad9 is necessary to invoke G2 cell cycle arrest. To confirm this they tested a double mutant of rad9 with DNA repair deficient-strain rad52 and found that the cell failed to arrest in G2 further proving that a functioning Rad9 gene is needed to induce G2 arrest. 
