The importance of the interaction between basic science and clinical practice has long been known but it has become even more evident in the past few decades with the impressive rate of development in the field of molecular genetics. This short article reviews molecular diagnosis of two different diseases for which scientific progress has immediately been translated into a dramatic improvement of the quality of medical care: the Fragile X Syndrome, paradigm of the new mutational mechanism of the unstable triplet repeats, and von Hippel-Lindau disease, a recent acquisition in the growing number of familial cancer syndromes.