The cat family was shown in 2005 to lack the T1R2 protein, one of two required for function of the sweetness sensory receptor; a deletion in the relevant gene (Tas1r2) causes a shift in the genetic reading frame, leading to transcription stopping early and no detectable mRNA or protein produced.[17] The other protein, T1R3, is present and identical to that of other animals, and the relevant taste buds are still present but inactive. Such a genetic marker found in the entire family and not other animals must be the result of a mutation in an early ancestor; as a deletion mutation it could not revert, and thus would be inherited by all descendants, as the evolutionary tree branched out. Some scientists now believe this is the root of the cat family's extremely specialized evolutionary niche as a hunter and carnivore. Their modified sense of taste would cause them to some degree to ignore plants, a large part of whose taste appeal derives from their high sugar content, in favor of a high-protein carnivorous diet, which would still stimulate their remaining taste receptors. The Felidae are the biological family of the cats; a member of this family is called a felid. The most familiar felid is the domestic cat, which first became associated with humans about 10,000 years ago; but the family includes all other wild cats, including the big cats. Extant felids belong to one of two subfamilies: Pantherinae (which includes the tiger, the lion, the jaguar, and the leopard), and Felinae (which includes the cougar, the cheetah, the lynxes, the ocelot, and the domestic cat). The first felids emerged during the Oligocene, about 25 million years ago. In prehistoric times, a third subfamily, known as Machairodontinae, included the "saber-toothed cats", such as the well known Smilodon. Other superficially cat-like mammals, such as the marsupial sabertooth Thylacosmilus or the Nimravidae, are not included i Felidae despite superficial similarities. Felids are the strictest carnivores of the 13 terrestrial families in the order Carnivora, although the three families of marine mammals comprising the superfamily Pinnipedia are as carnivorous as the felids. A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a polypeptide or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains. Genes hold the information to build and maintain an organism's cells and pass genetic traits to offspring. All organisms have many genes corresponding to various biological traits, some of which are immediately visible, such as eye color or number of limbs, and some of which are not, such as blood type, increased risk for specific diseases, or the thousands of basic biochemical processes that comprise life.[citation needed] The chemical structure of a four-base fragment of a DNA double helix. A modern working definition of a gene is "a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions, and or other functional sequence regions ".[1][2] Colloquial usage of the term gene (e.g. "good genes", "hair color gene") may actually refer to an allele: a gene is the basic instructiona sequence of nucleic acids (DNA or, in the case of certain viruses RNA), while an allele is one variant of that gene. Referring to having a gene for a trait is no longer the scientifically accepted usage. In most cases, all people would have a gene for the trait in question, but certain people will have a specific allele of that gene, which results in the trait variant. Further, genes code for proteins, which might result in identifiable traits, but it is the gene, not the trait, which is inherited.