Gene expression can be regulated at all of its stages. However, it is mainly regulated during transcription. In the first stage of protein synthesis, regulatory proteins known as transcription factors determine which genes get to be expressed or not. This process is highly controlled to allow the synthesis of only specific mRNA, which in turns synthesizes only specific proteins. Regulation of gene expression at the transcriptional level involves modifying the DNA-histone complex, which controls the genes being expressed. This form of gene regulation is known as epigenetics.