Gene expression can be controlled by repressors, promoters, activators, and enhancers. Expression is also controlled by epigenetic factors such as methylation of the DNA and the way the DNA is packaged. This can prevent the transcription machinery from accessing the DNA and can differ from cell type to cell type. In eukaryotic organisms the RNA can also be modified after transcription with a poly A tail or 5 prime cap that allows for slower degradation which usually leads to more protein product. Prokaryotes use operons like the LAC operon and the TRP operon to control gene express at the translational level as well as the transcriptional level. There are many ways in which genes are regulated. It is an entire field of study. These are just a few.