Based on an in vivo genome‐wide CRISPR/Cas9 screen in Trp53+/– heterozygous mice, we identified tumor suppressor genes that included the scaffold protein Axin1, the protein kinase A regulatory subunit gene Prkar1a, as well as the proof‐of‐concept genes Pten, Nf1, and Trp53 itself. This evidence concerns the gene TP53 and neoplasm.