Transcription factors control gene expression by interacting with DNA to promote or inhibit RNA synthesis. The family of basic helix-loop-helix (bHLH) transcription factors regulates genes involved in many aspects of growth and differentiation. The laboratory of Debby Good is using mice with targeted deletions of two members of this family, Nhlh1 and Nhlh2, to investigate the role of bHLH proteins in the developing nervous system. These photographs show that at 8 weeks of age, both normal and knockout mice are similar in weight and body structure. After 12 weeks, however, Nhlh2 knockout animals begin to display an obvious increase in abdominal fat and body weight.