Immunohistochemical staining using antibodies, two cell proliferation markers Ki‐67 and MCM2 and the squamous cell marker CK5 on serial sections showed nuclear positive reactivities of Ki‐67 and MCM2 and cytoplasmic positivity of CK5 throughout the tongue epithelium of the 4HT‐treated K(F/F) mice, suggesting squamous cell carcinoma in situ (SCC in situ/SCCIS). The gene discussed is KRT5; the disease is squamous carcinoma in situ.