Uclust 

UBLAST and USEARCH are algorithms designed to enable high-throughput, sensitive search of very large sequence databases. UBLAST searches for local alignments, USEARCH for global alignments. UBLAST and USEARCH are orders of magnitude faster than BLAST for some applications. UCLUST is a clustering algorithm that uses USEARCH as a subroutine to achieve exceptional high speed and sensitivity. The clustering method was implemented first, which is why the program name and manual tends to emphasize UCLUST rather than USEARCH. Some clustering algorithms like UCLUST use single-linkage clustering, constructing a transitive closure of sequences with a similarity over a particular threshold. UCLUST and CD-HIT use a greedy algorithm that identifies a representative sequence for each cluster and assigns a new sequence to that cluster if it is sufficiently similar to the representative; if a sequence is not matched then it becomes the representative sequence for a new cluster. The similarity score is often based on sequence alignment. Sequence clustering is often used to make a non-redundant set of representative sequences. (http://www.drive5.com/usearch/ , wikipedia)