Southern blot analysis of tissues from Nf1+/− mice with MPD. The 3.2-kb restriction fragment corresponds to the targeted Nf1 allele, and the 2.4-kb band is derived from the wild-type allele. DNA extracted from the bone marrows or spleens of five mice with leukemia (L) show absence or a marked reduction in the wild-type Nf1 allele compared to paired tail (T) DNA specimens.