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FGF5 is a negative regulator of hair formation. In a mouse lacking FGF5 (right), the hair is long compared with its heterozygous littermate (left). Transgenic mice with phenotypes that mimic aspects of a variety of human disorders, including Alzheimer's disease, atherosclerosis, diabetes, cystic fibrosis, and some type of cancers, have been generated. Their study may lead to the development of more effective treatments. (Courtesy of Gail Martin, from J.M. Hebert et al., Cell 78:1017–1025, 1994. © Elsevier.)