What distinguishes mammals from other vertebrates? Researchers from the Hospital del Mar Medical Research Institute (IMIM) have been trying to answer this question in collaboration with the researchers from the Universitat Pompeu Fabra (UPF). To do this, they analysed the 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 humans, it is estimated that they represent 2.5 percent of the genes that code for proteins. The study has been published in the journal Genome Biology and Evolution.