HIF1A and neoplasm: [23], [24]). The results demonstrate that HIF-1 activity was detected in most, if not all, cancers and papillomas as early as 9 weeks after MNU treatment (Figure 4A), indicating that HIF-1 activity is involved in papilloma formation and in the early stages of tumor development. These results strongly suggest that the Ras+HOL+ Tg mice would be useful for studying the initial events of tumor formation in vivo.