Id and development.

During development, it is obvious that enormous multiplication and diversification of cells is required to build a body plan from a single fertilized egg and that these two processes, proliferation and differentiation, must be coordinated properly. Id proteins, negative regulators of basic helix-loop-helix transcription factors, possess the ability to inhibit differentiation and to stimulate proliferation, and are useful molecules for investigating the mechanisms regulating development. In the past few years, our understanding of the roles of Id proteins has been substantially enhanced by the detailed investigation of genetically modified animals. The data have indicated that the functions of Id proteins in vivo are functionally related to those revealed by earlier work in cell culture systems. However, unexpected organs and cell types have also been found to require Id proteins for their normal development. This review looks at the advances made in our understanding of the in vivo functions of Id proteins. The topics discussed include neurogenesis, natural killer cell development, lymphoid organogenesis, mammary gland development and spermatogenesis.