TP53 and neoplasm: Nf1+/− mice only display MPNSTs when the monoallelic Nf1 deletion is combined with the loss of another tumor suppressor gene, such as p53, indicating that loss of these tumor suppressor alleles is the rate-limiting step in neurofibroma formation (Cichowski et al., 1999; Reilly et al., 2006; Rosenbaum et al., 1997; Vogel et al., 1999).