Figure 1. The high frequency of malignant conversion in FVB/N mice is demonstrated in vivo, in vitro, and in keratinocyte grafts. In vivo: After induction of skin tumors by an initiation-promotion protocol in FVB/N, BALB/c, and C57BL/6 mice, the conversion of papillomas to squamous cell carcinomas was compared in the mouse strains. In vitro: Primary skin keratinocytes isolated from each of the strains were used in a colony assay in cell culture to assess the frequency of carcinogen-induced progression to malignancy. Grafts: Human papillomavirus (HPV) type 16–immortalized keratinocyte cell lines from each mouse strain were grafted to the dorsal skin of athymic nude mice, and the frequency of carcinoma development in grafts was compared.