How Hox genes work
Each Hox gene is switched on in a particular place, along the body of the developing embryo. So cells get different combinations of chemical signals, depending on where they are in the embryo. Imagine a postman trying to deliver a letter - he needs to the street and house number to pinpoint the address. Both bits of information are vital for successful delivery but are useless without each other. Similarly, combinations of chemical signals let each cell know exactly where it is in the developing embryo.