MKI67 and neoplasm: In order to investigate the possible cause for the decreased number of adenomas in LSL-Kras, Rb1∆L/∆L mice we first tested the proliferation rate of the tumor cells between the two genotypes at six weeks and 12 weeks following the activation of Kras. We used Ki67 as a marker for proliferation and SA-β-gal staining as a marker for senescence in the tumors.