What distinguishes Homo sapiens from other living beings? And the group of mammals? What makes them different? Researchers analysed the already-sequenced genomes of 68 mammals and identified 6,000 families of genes that are only found in these animals. These are genes with no homologues outside mammals, in other words, they are not present in other hairless species. In humans, it is estimated that they represent 2.5% of the genes that code for proteins.