novo gene birth Novel orphan genes continually arise de novo from non-coding sequences. These novel genes may be sufficiently beneficial to be swept to fixation by selection. Or, more likely, they will fade back into the non-genic background. This latter option is supported by research in Drosophila showing that young genes are more likely go extinct. De novo genes were once thought to be a near impossibility due to the complex and potentially fragile intricacies of creating and maintaining functional polypeptides, but research from the past 10 years or so has found multiple examples of de novo genes, some of which are associated with important biological processes, particularly testes function in animals. De novo genes were also found in fungi and plants. For young orphan genes, it is sometimes possible to find homologous non-coding DNA sequences in sister taxa, which is generally accepted as strong evidence of de novo origin. However, the contribution of de novo origination to taxonomically-restricted genes of older origin, particularly in relation to the traditional gene duplication theory of gene evolution, remains contested. Logistically, de novo origination is much easier for RNA genes than protein-coding ones and Nathan H. Lents and colleagues recently reported the existence of several young microRNA genes on human chromosome 21. Duplication and divergence The duplication and divergence model for orphan genes involves a new gene being created from some duplication or divergence event and undergoing a period of rapid evolution where all detectable similarity to the originally duplicated gene is lost. While this explanation is consistent with current understandings of duplication mechanisms, the number of mutations needed to lose detectable similarity is large enough as to be a rare event, and the evolutionary mechanism by which a gene duplicate could be sequestered and diverge so rapidly remains unclear. Horizontal gene transfer Another explanation for how orphan genes arise is through a duplication mechanism called horizontal gene transfer, where the original duplicated gene derives from a separate, unknown lineage. This explanation for the origin of orphan genes is especially relevant in bacteria and archaea, where horizontal gene transfer is common. Protein characteristics Orphans genes tend to be very short (~6 times shorter than mature genes), and some are weakly expressed, tissue specific and simpler in codon usage and amino acid composition. Orphan genes tend to encode more intrinsically disordered proteins, although some structure has been found in one of the best characterized