The answer is that the cells are epigenetically different. To understand this, it is first essential to clarify the central dogma in molecular biology. Your genome consists of about three billion base pairs of DNA, containing about 25,000 genes. To make a protein from a gene, enzymes bind to the gene and ‘transcribe’ it into an intermediate called mRNA. A second set of enzymes then ‘translate’ the mRNA into protein. Consider a cell in your brain and one in your liver.