Genomics is the study of an organism's entire genome. A major branch of genomics is concerned with sequencing the genomes of various organisms. It has been used to sequence the entire genome of the mouse, Xenopus frog and even humans. Such knowledge of complete genomes has created the possibility for functional genomics, the study of the result of patterns of gene expression under various conditions. Expression of thousands of genes can be measured simultaneously using microarrays and analysed by sophisticated computational tools. For example, microarrays can be used to identify disease genes by comparing gene expression in diseased and normal cells and many researchers within the Department are using genomics for just that reason.