Transposons or Transposable elements (TEs) are "mobile genes" capable of mobilization from one genomic location to another through non-homologous recombination. As this movement is mediated by its own proteins and does not contribute to the survival of the host that it inhabits, they are known as selfish genomic parasites. Despite their capacity for transposition inside genomes, they can frequently transpose the species boundaries and consequently migrate from one species to another. Such phenomenon is called Horizontal Transposons Transfer. HTT was first discovered by Daniels et al. (1984) when analysing a P element that was transferred from Drosophila willistoni to D. melanogaster. Since then, many more cases have been documented in the literature. Moreover, in the last years, such discoveries have been boosted by the unprecedented amount of new genomes available. Despite the recognition of HTT as a common phenomenon in recent years, it is still difficult to draw major conclusions about HTT patterns, such as where in the tree of life these cases are more frequently found. This is mainly due to the historical bias and lack of studies in many taxa. To date, there has been no easy way to visualise each TE or host species, and should be further analysed in order to provide a more comprehensive view of such phenomena. Based on these concerns, we developed the HTT database to keep an updated repository of HTT events in all eukaryotes, allowing not only TE specialists to add new events and search the database, but also non-specialists. Moreover, we expanded the database to include Horizontal-Virus Transfer also known as endogenization events which is characterized by the stable integration a viral genomic fragment into the host genome.
