gene expression is controlled by several things:
1. Transcription factors (repressors and activators)
2. DNA epigenetic changes
3. Histone epigenetic changes
4. Enhancers (long-range)
In addition, once a genomic transcript is generated, it can be spliced, truncated, degraded in so many ways that it changes eventual protein levels. Also, protein can be controlled in several ways (ribosomal processing, tagging for degradation etc) during and after translation.
And this is very basic idea, there is a lifetime of complexity to talk about here.
So. Now that you have your first ever question on bioforum answered (or not), mail yourself your username and password so you don't forget them, and then come back soon to update us on how it all worked out. That's how you build Karma in science.