All living organisms share certain features, presumably due to our descent from a single common ancestor. These shared features are called universal homologies, and include our cellular organization (all living things are made of cells), our use of the molecule ATP to do work at the cellular level, and our common genetic code. The genetic code is a set of rules governing how the subunits of a gene (the nucleotides) correspond to the subunits of a protein (amino acids). Biologists use an understanding of the genetic code to study evolutionary relationships among different groups of organisms. They do this by comparing the proteins that are produced, or by looking for differences in the genes themselves. Genetic analyses may clarify differences between populations, such as the presence of the non-functioning CCR5 gene (discussed above) that confers resistance to HIV. Some populations (for example, Icelanders) have a high frequency of the gene, and others (such as Kenyans) don’t have this version of the gene at all–this difference is evidence for evolution in humans. Biochemical comparisons can also highlight what genes have been maintained, or conserved, over time. For example, a gene essential to sperm production is present in all sperm-producing animals—from sea anemones to leeches to humans. These animals share a common ancestor that lived 600 million years ago; thus, this highly conserved sperm-producing gene must have been pretty important during animal evolution.