NF1 and neoplasm: Using single nucleotide polymorphism and microsatellite analysis as previously described [15], we found that tumors from mutagenized Nf1+/- mice commonly lose the wildtype Nf1 allele, rendering these tumors null for Nf1. Tumor formation in irradiated Nf1+/- mice is driven by complete loss of Nf1, which is also a hallmark of tumor formation in NF1 patients [28] and thus represents an early and necessary event.