To ascertain whether xis is also involved in 89K excision, the excision of 89K was examined in an xis deletion mutant of strain 05ZYH33 (named 05ZYH33Dxis). As shown in Fig. 3B , PCR analysis of the excised circular 89K and the repaired chromosome using specific primers both gave faint but still detectable signals in the Dxis mutant, suggesting that there is a very low level of Xisindependent 89K excision. qPCR analysis revealed that xis deletion decreased the presence of attP to~0.0012%, while functional complementation of xis restored the excision frequency to~0.029%. These results demonstrated that xis stimulates, but is not essential for, 89K excision from the chromosome.