Mutant p53 proteins not only lose their tumor-suppressor function but some acquire oncogenic gain of function (GOF). The published mutp53 knock-in (KI) alleles (R172H, R270H, R248W) manifest GOF by broader tumor spectrum and more metastasis compared with the p53-null allele, but do not shorten survival. However, whether GOF also occurs with other mutations and whether they are all biologically equal is unknown. To answer this, we created novel humanized mutp53 KI mice harboring the hot spot alleles R248Q and G245S. Intriguingly, their impact was very different. Compared with p53-null mice, R248Q/ À mice had accelerated onset of all tumor types and shorter survival, thus unprecedented strong GOF. In contrast, G245S/ À mice were similar to null mice in tumor latency and survival. This was associated with a twofold higher T-lymphoma proliferation in R248Q/ À mice compared with G245S/ À and null mice. Moreover, R248Q/ À hematopoietic and mesenchymal stem cells were expanded relative to G245S/ À and null mice, the first indication that GOF also acts by perturbing pretumorous progenitor pools. Importantly, these models closely mirror Li-Fraumeni patients who show higher tumor numbers, accelerated onset and shorter tumor-free survival by 10.5 years when harboring codon R248Q mutations as compared with Li-Fraumeni patients with codon G245S mutations or p53 deletions/loss. Conversely, both KI alleles caused a modest broadening of tumor spectrum with enhanced Akt signaling compared with null mice. These models are the first in vivo proof for differential oncogenic strength among p53 GOF alleles, with genotype-phenotype correlations borne out in humans. p53 is mutated in over 50% of human cancers. 1 In response to oncogenic mutations or DNA damage, wtp53 rapidly stabilizes in the nucleus, triggering a transcriptional program of cell cycle arrest, DNA repair, senescence, autophagy and apoptosis. The vast majority (95%) of p53 mutations in human cancer are missense mutations (mutp53). They are broadly distributed within the DNA-binding domain (aa 102-292) with 6 hot spots at codons 175, 245, 248, 249, 273 and 282, and generate conformationally aberrant proteins with impaired or abrogated transcriptional function and abrogated proapoptotic mitochondrial actions. 1,2 Accumulating evidence indicates multiple newly acquired active roles for mutp53 proteins in promoting tumorigenesis. [3][4][5] Knock-in (KI) mouse models expressing the hot spot mutant alleles R172H and R270H (equivalent to human R175H and R273H) from the endogenous promoter provided definitive proof that at least certain p53 mutants exhibit gain of functions (GOFs). 6,7 They established that these mutp53 proteins cause a broader tumor spectrum including carcinomas and progression of certain tumor types to a more invasive and metastatic phenotype compared with tumors of p53 À / À or p53 þ / À mice. Similarly, three HUPKI (Humanized p53 Knock In) mouse models (harboring R175H, R273H and R248W) show broader tumor spectrum compared with null mice, confirming a GOF fo...