We first labeled an Nf1+/−;Tp53+/− mouse model of malignant peripheral nerve sheath tumor (MPNST), incidentally finding a second intra-abdominal histiosarcoma, which in both tumor types is caused by a stochastic loss of wild-type Nf1 and Tp53 alleles. The gene discussed is NF1; the disease is malignant peripheral nerve sheath tumor.