7,12-Dimethylbenz[a]anthracene (DMBA)-induced carcinogenesis of the hamster buccal pouch has been an excellent model for the study of squamous cell carcinogenesis in human head and neck cancer. Using a differential hybridization of cDNA cloning technique, we isolated a cDNA clone that is expressed in N-ras-transformed PA-1 cells but poorly expressed in non-tumorigenic PA-1 cells; the cDNA codes for the human ribosomal S2 gene product. To define the involvement of S2 gene expression during carcinogenesis in this animal model, we used in situ hybridization technique with non-radioactive digoxigenin-11-dUTP-labeled cDNA. S2 gene was expressed at low levels in basal and suprabasal cell layers of the epidermis in the control, but showed marked elevation throughout the epidermis other than the keratin layer in samples treated for 4 or 8 weeks; S2 was highly expressed in all malignant squamous cell carcinoma cells resulting from DMBA treatment for 16 weeks. As tumors progress from normal epithelium to squamous cell carcinomas, mRNA of the S2 gene was not only elevated sequentially, but also demonstrated the marked heterogeneity among transformed populations, particularly in dysplastic lesions and squamous cell carcinomas. The S2 gene was expressed in a stage-specific manner in the hamster tumor model; S2 could be useful as a neoplastic marker for the detection of certain epithelial origin of tumors and premalignant lesions as well.