A major focus of modern molecular biology is the isolation and analysis of genes based upon associated phenotypes. The strategies for “positional cloning,” where a gene is cloned, analyzed and the basic underlying biology uncovered based upon its chromosomal map location alone, form the basis of much modern molecular genetics research and has made possible the isolation, characterization and understanding of genes responsible for many human hereditary, malignant and degenerative diseases. The success of this type of phenotype-based gene hunting has been dramatically illustrated through the cloning of genes responsible for Duchenne’s muscular dystrophy (1), cystic fibrosis (2) and neurofibromatosis type 1 (3), among many others.