One of the central problems of biological sciences is to understand the manner in which the single-celled fertilized egg (the zygote) develops into a multicellular organism with a vast range of different cell types, each of which forms at the appropriate time and place relative to other cells. As will be discussed in this chapter, the regulation of gene transcription by specific transcription factors plays a key role in this process. A number of these transcription factors are expressed at specific times and places during embryonic development and play a central role in this process. Such regulation of transcription factor synthesis contrasts with the regulation of transcription factor activity that occurs in response to cellular signaling pathways, as discussed in the previous chapter.<\/p>\n\n\n\n