Goltz syndrome (focal dermal hypoplasia) is an X-linked dominant, multisystem birth defect with lethality for male embryos. The hypoplastic skin lesions follow Blaschko's lines and often show herniation of subcutaneous fatty tissue. Extracutaneous defects mainly involve the brain, the bones, the teeth, and the eyes. All of these anomalies show a segmental arrangement reflecting functional X-chromosome mosaicism. The present contribution tells the story how the molecular cause of Goltz syndrome was investigated and found by the group of the geneticist Karl–Heinz Grzeschik from Marburg, Germany.