The basic unit of heredity; the sequence of DNA that encodes all the information to make a protein. Structurally, a gene is formed by three regions: a regulatory region called the promoter juxtaposed to the coding region containing the protein sequence, and a “3’ tail” sequence. In mammalian cells, the promoter is a complex region containing binding sites for many proteins that regulate gene expression. A gene may be “activated” or “switched on” to make protein – this activation is referred to as gene expression - by these proteins which control when, where and how much protein is expressed from the gene. In the human genome, there are an estimated 100,000 genes. Some of these are evolutionarily related and form“gene families” that express related proteins. There are also genes that no longer make a protein; these defective remnants of evolution are called pseudogenes.