Gene regulation accounts for changes during development as well as differences among organisms that have similar genes. Cells can use energy and materials efficiently. Gene regulation may enable us to treat and prevent diseases that were previously unbeatable. More complex, variable and involves more steps and interactions than in prokaryotes. Regulation can occur before transcription, after transcription, or after translation. A nuclear membrane separates these processes. So each process can be regulated separately. Operons are common in bacteria but very rare in eukaryotic cells. Groups of genes with related functions may be scattered on different chromosomes and controlled by multiple factors. DNA in eukaryotes may never be transcribed, and even less is ultimately translated into proteins.