Bioinformatics was brought into the spotlight in the late 1990s through the Human Genome Project. With the rapid accumulation of completed genomes, it was soon realized that for the vast majority of the newly identified genes and other functional regions of the genomes there were no other biological data. One way of inferring biological function is through homology: Because homologous genes have a common evolutionary descent, they are likely to have the same biological function. A large number of bioinformatics tools have been designed for rapidly and accurately comparing sequences of genes or proteins, comparing gene sequences with genomes, and comparing genomes. Two widely used tools for sequence alignment and homology searches, BLAST and ClustalW, are introduced here.