MKI67 and neoplasm: Given that AID expression in pancreas promotes mutations and DNA damage without leading to tumor development, we sought for the evidence of precancerous cells and found that pancreas from aged R26AID+/KIp48-CRE+/KI mice contained more proliferating cells, as assessed by Ki67 staining, than control pancreas (Fig 4A), indicating that pancreatic AID expression leads to an abnormal rate of cell division.