Consistent with our data, mouse models support a complex relationship between cellular genotype and phenotype in neurofibromatosis type 1—the genetic background of the mouse, the identity of the cell in which NF1 is inactivated, the presence of cooperating somatic mutations and the status of NF1 function in neighboring cells, all appear to affect tumor development30,31. This evidence concerns the gene NF1 and neurofibromatosis type 1.