The document summarizes regulation of gene expression in prokaryotes and eukaryotes. In prokaryotes, genes are organized into operons containing a promoter, operator, and multiple coding sequences. The lac operon in E.coli contains genes regulated by the repressor protein LacI and induced by lactose. In eukaryotes, gene expression is regulated at multiple levels including transcription, RNA processing, transport, translation and degradation. Mechanisms include histone modification, DNA methylation, hormone signaling, polyadenylation, splicing, transport and protein degradation.