Most mammalian species have between ten and fifteen types of Toll-like receptors. Thirteen TLRs (TLR1 to TLR13) have been identified in humans and mice, and similar ones have been found in other mammalian species. TLRs are not identical in all mammals. For example, a gene coding for a protein like TLR10 in humans is present in mice, but seems to have been damaged at some time by a retrovirus. On the other hand, mice express TLRs 11, 12, and 13, none of which is present in humans. Other mammals may express TLRs that are not found in humans. Non-mammalian species may have TLRs different from mammals.