In HCT-116 xenografts, we observed only slightly fewer PCNA-positive cells than Ki67-positive cells (53.91 ± 3.18 % vs. 68.50 ± 5.52 %, p  =  0.0003), yet only a small fraction of tumor cells in DiFi xenografts were PCNA-positive (13.59 ± 1.48 % vs. 72.37 ± 12.64 %, p  =  0.0003). The gene discussed is MKI67; the disease is neoplasm.