In response to both GH and IGF-I, periosteal new bone formation leads to an increase in skeletal growth, especially at the level of the mandible (prognathism); jaw thickening, teeth separation, frontal bossing, malocclusion, and nasal bone hypertrophy are the usual facial bony deformities seen in acromegaly (Figure 3). The gene discussed is IGF1; the disease is acromegaly.