Gene prediction is commonly performed through a combination of what are known as extrinsic and intrinsic searches.[5] For the extrinsic search, the input DNA sequence is run through a large database of sequences whose genes have been previously discovered and their locations annotated. A number of the sequence's genes can be identified by determining which strings of bases within the sequence are homologous to known gene sequences. However, given the limitation in size of the database of known and annotated gene sequences, not all the genes in a given input sequence can be identified through homology alone.