Horizonal gene transfer, i.e. the process of acquiring genes from foreign species, is a frequent phenomenon among microbial species. It is considered a strong evolutionary force allowing rapid adaptation to changing environmental demands. The transferred pieces of DNA often comprise a large number of genes that are found in contiguous regions called Genomic Islands (GIs). We have implemented the program SIGI-HMM that predicts GIs and the putative donor of each individual alien gene, by the use of a HMM that takes into account codon usage.