The accurate transcription of 5S RNA genes when injected into the nucleus of Xenopus oocytes or when added to an in vitro transcription system has allowed identification of the DNA sequences and one of the protein factors required for 5S RNA synthesis. Moreover, 5S RNA genes as part of intact chromosomes maintain a transcriptionally regulated state when injected into Xenopus oocyte nuclei. A detailed picture of the developmental regulation of 5S RNA gene expression is now emerging.