[Orphan genes: function, evolution, composition].

The origin of orphan genes and the function they perform remain an enigmatic problem that modern molecular biology has to solve. The recently developed PHOG database helped to shed light on some aspects of the evolution of these genes. Fast evolution is probably the main factor that shapes this evolutionary process. The speed of molecular evolution reflects the degree of gene conservation, though exceptions from this rule also contribute to the dynamic process of genome evolution during speciation events. The existence of the great number of orphan genes is an artifact of insufficient sequencing. If the DNA of all organisms living on Earth were sequences, then the number of orphan genes will be greatly reduced, giving the way to the genes specific for particular taxonomic groups.