To investigate this possibility, we first examined whether Ras could up-regulate the expression of ribosomal proteins in four different mouse keratinocyte cell lines: BalMk2 normal mouse keratinocytes with wild-type Ras, 308 benign mouse skin papilloma cells [42], CH72-T3 malignant mouse skin squamous cell carcinoma cells [43], and CC4A malignant mouse skin carcinoma cells all carrying an H-Ras mutation at codon 61 [44]. This evidence concerns the gene HRAS and skin papilloma.