Although neurofibromatosis 1 (NF1) is a common genetic disorder, its expression is highly variable and unpredictable. Here, we review the genotype-phenotype correlations in NF1. Some NF1 patients with a given NF1 mutation may develop very severe disease while others with the same mutation have only mild symptoms, even in the same family. The clinical variability may therefore be due to both modifier genes and environmental factors. Recent targeted strategies have identified several interesting candidate modifier genes. (By Dr. Eric Pasmant, http://jmg.bmj.com/content/49/8/483.abstract?etoc )