Differential gene expression is required to establish and maintain specific developmental states in higher plants. For example, an anther has at least 11 000 diverse mRNAs that are absent from the polysomes of other organ systems, and the root has at least 7000 organ-specific mRNAs. Both transcriptional and post-transcriptional processes regulate the sequence composition and prevalence distribution of each developmental-specific mRNA set. Soybean seed protein genes represent an excellent example of a highly regulated gene set. These genes are temporally and spatially regulated during embryogenesis, and are either inactive or expressed at low levels in mature plant organ systems. Gene transfer experiments indicate that soybean seed protein genes retain their developmental-specific expression programme in transformed tobacco plants. In addition, large polygenic clusters can be transferred from soybean to tobacco, and the expression pattern of each gene within the cluster is maintained in the foreign cell environment. Although the DNA sequences and cellular factors required to control seed protein gene expression are not yet known, gene transfer studies and emerging DNA binding protein technology should facilitate their identification in the near future.