"What is that 95 percent? Some is junk--remnants of pseudogenes inactivated by evolution.fn4,3 But buried in that are the keys to the kingdom, the instruction manual for when to transcribe particular genes, the on/off switches for gene transcription. A gene doesn't "decide" when to be photocopied into RNA, to generate its protein. Instead, before the start of the stretch of DNA coding for that gene is a short stretch called a promoterfn5--the "on" switch. What turns the promoter switch on? Something called a transcription factor (TF) binds to the promoter. This causes the recruitment of enzymes that transcribe the gene into RNA. Meanwhile, other transcription factors deactivate genes."