Second, you have to know that in 2000, scientists have mapped the human genome. This means that they worked to see the general picture of a layout of the genes of a representative human – an estimate of the number of genes, and a very general perspective of what those genes are for. This is not, by any measure, an accurate and detailed map. This is because up to now, scientists are revising the number of genes that make up a human. We are still finding out what those many genes code for in our makeup, since there is no one gene for anything, and also because one gene can contribute to many traits and susceptibilities. Scientists have since been building knowledge on some of the genes and what they are for. Among those many genes, one is CCR5, which has been found to open the cells for HIV infection to enter the body.