The same GI can occur in distantly related species as a result of various types of horizontal gene transfer (transformation, conjugation, transduction).
Two major problems with these methods are that false predictions can occur due to natural variation in the genome (sometimes due to highly expressed genes) and that horizontally transferred DNA will ameliorate (change to the host genome) over time; therefore, limiting predictions to only recently acquired GIs.
Comparative genomics based methods try to identify regions that show signs that they have been horizontally transferred using information from several related species.
These "islands" are characterised by their large size(>10 Kb), their frequent association with tRNA-encoding genes and a different G+C content compared with the rest of the genome.
Some genomic islands, including those adjacent to integrative and conjugative elements (ICEs), can excise themselves spontaneously from the chromosome and can be transferred to other suitable recipients.