Generalizing the control process for embryonic genes.

Embryonic genes are considered as a separate subset of genes with unique chromatin properties. There is a problem of defining the duration of perturbations of embryonic gene activity that has been chemically induced and the normal relatively longer lasting changes that occur during differentiation. This problem may be related to unique properties of the chromatin of embryonic genes. Methylation of DNA is thought to be only one level of control and the superstructure of chromatin involving heterochromatin is of equal importance to embryonic gene expression. Proto-oncogenes are considered to be embryonic type genes whose activities are regulated under the same mechanisms by which other embryonic genes are regulated. Control aspects are discussed in the light of i) repressor-derepressor and blocking-deblocking mechanisms, ii) activator genes, pseudogenes, LINES, SINES, v-type position effects, iii) effects of ethionine, and iv) steroid hormone effects especially with respect to a subset of repeated rRNA genes which are considered to be structured in embryonic type chromatin.