An Arf(GFP/GFP) reporter mouse reveals that the Arf tumor suppressor monitors latent oncogenic signals in vivo.

Understanding how the Arf tumor suppressor is activated in response to abnormally elevated mitogenic signaling thresholds is a particularly vexing problem. Studies of a knock-in mouse strain in which sequences encoding green fluorescent protein were substituted for those encoding p19Arf argue that the Arf gene responds to latent oncogenic signals in vivo to eliminate incipient cancer cells.