The genes themselves are complex structures, having a starting code (promoter) built into the beginning of each gene and an exit code (terminator) at the end. Amid the coding sequences (exons) there are intervening non-coding sequences (introns), which need to be removed before use. In general, if a primitive organism has a particular gene, then organisms of increasing complexity will contain a number of related genes in proportion to their position on the evolutionary scale. This suggests that with time, genes are often duplicated and then evolve their sequences separately.