Jin et al. then stained the lungs for the proliferation marker Ki67 and showed that the lungs from mice grown in specific-pathogen free conditions were more heavily proliferating (i.e., Ki67-positive) compared to the lungs of the germ-free grown mice, demonstrating that the presence of commensal bacteria in the specific pathogen free grown mice exacerbated the proliferation of tumor cells. The gene discussed is MKI67; the disease is neoplasm.